SQL Syntax
==========

syntax

SQL

This chapter describes the syntax of SQL. It forms the foundation for
understanding the following chapters which will go into detail about how
SQL commands are applied to define and modify data.

We also advise users who are already familiar with SQL to read this
chapter carefully because it contains several rules and concepts that
are implemented inconsistently among SQL databases or that are specific
to PostgreSQL.

Lexical Structure {#sql-syntax-lexical}
=================

token

SQL input consists of a sequence of commands. A command is composed of a
sequence of tokens, terminated by a semicolon (";"). The end of the
input stream also terminates a command. Which tokens are valid depends
on the syntax of the particular command.

A token can be a key word, an identifier, a quoted identifier, a literal
(or constant), or a special character symbol. Tokens are normally
separated by whitespace (space, tab, newline), but need not be if there
is no ambiguity (which is generally only the case if a special character
is adjacent to some other token type).

For example, the following is (syntactically) valid SQL input:

    SELECT * FROM MY_TABLE;
    UPDATE MY_TABLE SET A = 5;
    INSERT INTO MY_TABLE VALUES (3, 'hi there');

This is a sequence of three commands, one per line (although this is not
required; more than one command can be on a line, and commands can
usefully be split across lines).

Additionally, comments can occur in SQL input. They are not tokens, they
are effectively equivalent to whitespace.

The SQL syntax is not very consistent regarding what tokens identify
commands and which are operands or parameters. The first few tokens are
generally the command name, so in the above example we would usually
speak of a "SELECT", an "UPDATE", and an "INSERT" command. But for
instance the `UPDATE` command always requires a SET token to appear in a
certain position, and this particular variation of `INSERT` also
requires a VALUES in order to be complete. The precise syntax rules for
each command are described in [???](#reference).

Identifiers and Key Words {#sql-syntax-identifiers}
-------------------------

identifier

syntax of

name

syntax of

key word

syntax of

Tokens such as SELECT, UPDATE, or VALUES in the example above are
examples of key words, that is, words that have a fixed meaning in the
SQL language. The tokens MY\_TABLE and A are examples of identifiers.
They identify names of tables, columns, or other database objects,
depending on the command they are used in. Therefore they are sometimes
simply called "names". Key words and identifiers have the same lexical
structure, meaning that one cannot know whether a token is an identifier
or a key word without knowing the language. A complete list of key words
can be found in [???](#sql-keywords-appendix).

SQL identifiers and key words must begin with a letter (`a`-`z`, but
also letters with diacritical marks and non-Latin letters) or an
underscore (`_`). Subsequent characters in an identifier or key word can
be letters, underscores, digits (`0`-`9`), or dollar signs (`$`). Note
that dollar signs are not allowed in identifiers according to the letter
of the SQL standard, so their use might render applications less
portable. The SQL standard will not define a key word that contains
digits or starts or ends with an underscore, so identifiers of this form
are safe against possible conflict with future extensions of the
standard.

identifierlength The system uses no more than `NAMEDATALEN`-1 bytes of
an identifier; longer names can be written in commands, but they will be
truncated. By default, `NAMEDATALEN` is 64 so the maximum identifier
length is 63 bytes. If this limit is problematic, it can be raised by
changing the `NAMEDATALEN` constant in `src/include/pg_config_manual.h`.

case sensitivity of SQL commands Key words and unquoted identifiers are
case insensitive. Therefore:

    UPDATE MY_TABLE SET A = 5;

can equivalently be written as:

    uPDaTE my_TabLE SeT a = 5;

A convention often used is to write key words in upper case and names in
lower case, e.g.:

    UPDATE my_table SET a = 5;

quotation marks and identifiers There is a second kind of identifier:
the delimited identifier or quoted identifier. It is formed by enclosing
an arbitrary sequence of characters in double-quotes (`"`). A delimited
identifier is always an identifier, never a key word. So `"select"`
could be used to refer to a column or table named "select", whereas an
unquoted `select` would be taken as a key word and would therefore
provoke a parse error when used where a table or column name is
expected. The example can be written with quoted identifiers like this:

    UPDATE "my_table" SET "a" = 5;

Quoted identifiers can contain any character, except the character with
code zero. (To include a double quote, write two double quotes.) This
allows constructing table or column names that would otherwise not be
possible, such as ones containing spaces or ampersands. The length
limitation still applies.

Quoting an identifier also makes it case-sensitive, whereas unquoted
names are always folded to lower case. For example, the identifiers
`FOO`, `foo`, and `"foo"` are considered the same by PostgreSQL, but
`"Foo"` and `"FOO"` are different from these three and each other. (The
folding of unquoted names to lower case in PostgreSQL is incompatible
with the SQL standard, which says that unquoted names should be folded
to upper case. Thus, `foo` should be equivalent to `"FOO"` not `"foo"`
according to the standard. If you want to write portable applications
you are advised to always quote a particular name or never quote it.)

Unicode escape

in identifiers

A variant of quoted identifiers allows including escaped Unicode
characters identified by their code points. This variant starts with
`U&` (upper or lower case U followed by ampersand) immediately before
the opening double quote, without any spaces in between, for example
`U&"foo"`. (Note that this creates an ambiguity with the operator `&`.
Use spaces around the operator to avoid this problem.) Inside the
quotes, Unicode characters can be specified in escaped form by writing a
backslash followed by the four-digit hexadecimal code point number or
alternatively a backslash followed by a plus sign followed by a
six-digit hexadecimal code point number. For example, the identifier
`"data"` could be written as

    U&"d\0061t\+000061"

The following less trivial example writes the Russian word "slon"
(elephant) in Cyrillic letters:

    U&"\0441\043B\043E\043D"

If a different escape character than backslash is desired, it can be
specified using the `UESCAPE`UESCAPE clause after the string, for
example:

    U&"d!0061t!+000061" UESCAPE '!'

The escape character can be any single character other than a
hexadecimal digit, the plus sign, a single quote, a double quote, or a
whitespace character. Note that the escape character is written in
single quotes, not double quotes, after `UESCAPE`.

To include the escape character in the identifier literally, write it
twice.

Either the 4-digit or the 6-digit escape form can be used to specify
UTF-16 surrogate pairs to compose characters with code points larger
than U+FFFF, although the availability of the 6-digit form technically
makes this unnecessary. (Surrogate pairs are not stored directly, but
are combined into a single code point.)

If the server encoding is not UTF-8, the Unicode code point identified
by one of these escape sequences is converted to the actual server
encoding; an error is reported if that\'s not possible.

Constants {#sql-syntax-constants}
---------

constant

There are three kinds of implicitly-typed constants in PostgreSQL:
strings, bit strings, and numbers. Constants can also be specified with
explicit types, which can enable more accurate representation and more
efficient handling by the system. These alternatives are discussed in
the following subsections.

### String Constants {#sql-syntax-strings}

character string

constant

quotation marks escaping A string constant in SQL is an arbitrary
sequence of characters bounded by single quotes (`'`), for example
`'This is a string'`. To include a single-quote character within a
string constant, write two adjacent single quotes, e.g.,
`'Dianne''s horse'`. Note that this is *not* the same as a double-quote
character (`"`).

Two string constants that are only separated by whitespace *with at
least one newline* are concatenated and effectively treated as if the
string had been written as one constant. For example:

    SELECT 'foo'
    'bar';

is equivalent to:

    SELECT 'foobar';

but:

    SELECT 'foo'      'bar';

is not valid syntax. (This slightly bizarre behavior is specified by
SQL; PostgreSQL is following the standard.)

### String Constants with C-Style Escapes {#sql-syntax-strings-escape}

escape string syntax

backslash escapes

PostgreSQL also accepts "escape" string constants, which are an
extension to the SQL standard. An escape string constant is specified by
writing the letter `E` (upper or lower case) just before the opening
single quote, e.g., `E'foo'`. (When continuing an escape string constant
across lines, write `E` only before the first opening quote.) Within an
escape string, a backslash character (`\`) begins a C-like backslash
escape sequence, in which the combination of backslash and following
character(s) represent a special byte value, as shown in
[table\_title](#sql-backslash-table).

  Backslash Escape Sequence                       Interpretation
  ----------------------------------------------- --------------------------------------------------
  `\b`                                            backspace
  `\f`                                            form feed
  `\n`                                            newline
  `\r`                                            carriage return
  `\t`                                            tab
  `\o`, `\oo`, `\ooo` (o = 0NDASH7)               octal byte value
  `\xh`, `\xhh` (h = 0NDASH9, ANDASHF)            hexadecimal byte value
  `\uxxxx`, `\Uxxxxxxxx` (x = 0NDASH9, ANDASHF)   16 or 32-bit hexadecimal Unicode character value

  : Backslash Escape Sequences

Any other character following a backslash is taken literally. Thus, to
include a backslash character, write two backslashes (`\\`). Also, a
single quote can be included in an escape string by writing `\'`, in
addition to the normal way of `''`.

It is your responsibility that the byte sequences you create, especially
when using the octal or hexadecimal escapes, compose valid characters in
the server character set encoding. A useful alternative is to use
Unicode escapes or the alternative Unicode escape syntax, explained in
[String Constants with Unicode Escapes](#sql-syntax-strings-uescape);
then the server will check that the character conversion is possible.

> **Caution**
>
> If the configuration parameter [???](#guc-standard-conforming-strings)
> is `off`, then PostgreSQL recognizes backslash escapes in both regular
> and escape string constants. However, as of PostgreSQL 9.1, the
> default is `on`, meaning that backslash escapes are recognized only in
> escape string constants. This behavior is more standards-compliant,
> but might break applications which rely on the historical behavior,
> where backslash escapes were always recognized. As a workaround, you
> can set this parameter to `off`, but it is better to migrate away from
> using backslash escapes. If you need to use a backslash escape to
> represent a special character, write the string constant with an `E`.
>
> In addition to `standard_conforming_strings`, the configuration
> parameters [???](#guc-escape-string-warning) and
> [???](#guc-backslash-quote) govern treatment of backslashes in string
> constants.

The character with the code zero cannot be in a string constant.

### String Constants with Unicode Escapes {#sql-syntax-strings-uescape}

Unicode escape

in string constants

PostgreSQL also supports another type of escape syntax for strings that
allows specifying arbitrary Unicode characters by code point. A Unicode
escape string constant starts with `U&` (upper or lower case letter U
followed by ampersand) immediately before the opening quote, without any
spaces in between, for example `U&'foo'`. (Note that this creates an
ambiguity with the operator `&`. Use spaces around the operator to avoid
this problem.) Inside the quotes, Unicode characters can be specified in
escaped form by writing a backslash followed by the four-digit
hexadecimal code point number or alternatively a backslash followed by a
plus sign followed by a six-digit hexadecimal code point number. For
example, the string `'data'` could be written as

    U&'d\0061t\+000061'

The following less trivial example writes the Russian word "slon"
(elephant) in Cyrillic letters:

    U&'\0441\043B\043E\043D'

If a different escape character than backslash is desired, it can be
specified using the `UESCAPE`UESCAPE clause after the string, for
example:

    U&'d!0061t!+000061' UESCAPE '!'

The escape character can be any single character other than a
hexadecimal digit, the plus sign, a single quote, a double quote, or a
whitespace character.

To include the escape character in the string literally, write it twice.

Either the 4-digit or the 6-digit escape form can be used to specify
UTF-16 surrogate pairs to compose characters with code points larger
than U+FFFF, although the availability of the 6-digit form technically
makes this unnecessary. (Surrogate pairs are not stored directly, but
are combined into a single code point.)

If the server encoding is not UTF-8, the Unicode code point identified
by one of these escape sequences is converted to the actual server
encoding; an error is reported if that\'s not possible.

Also, the Unicode escape syntax for string constants only works when the
configuration parameter [???](#guc-standard-conforming-strings) is
turned on. This is because otherwise this syntax could confuse clients
that parse the SQL statements to the point that it could lead to SQL
injections and similar security issues. If the parameter is set to off,
this syntax will be rejected with an error message.

### Dollar-Quoted String Constants {#sql-syntax-dollar-quoting}

dollar quoting

While the standard syntax for specifying string constants is usually
convenient, it can be difficult to understand when the desired string
contains many single quotes or backslashes, since each of those must be
doubled. To allow more readable queries in such situations, PostgreSQL
provides another way, called "dollar quoting", to write string
constants. A dollar-quoted string constant consists of a dollar sign
(`$`), an optional "tag" of zero or more characters, another dollar
sign, an arbitrary sequence of characters that makes up the string
content, a dollar sign, the same tag that began this dollar quote, and a
dollar sign. For example, here are two different ways to specify the
string "Dianne\'s horse" using dollar quoting:

    $$Dianne's horse$$
    $SomeTag$Dianne's horse$SomeTag$

Notice that inside the dollar-quoted string, single quotes can be used
without needing to be escaped. Indeed, no characters inside a
dollar-quoted string are ever escaped: the string content is always
written literally. Backslashes are not special, and neither are dollar
signs, unless they are part of a sequence matching the opening tag.

It is possible to nest dollar-quoted string constants by choosing
different tags at each nesting level. This is most commonly used in
writing function definitions. For example:

    $function$
    BEGIN
        RETURN ($1 ~ $q$[\t\r\n\v\\]$q$);
    END;
    $function$

Here, the sequence `$q$[\t\r\n\v\\]$q$` represents a dollar-quoted
literal string `[\t\r\n\v\\]`, which will be recognized when the
function body is executed by PostgreSQL. But since the sequence does not
match the outer dollar quoting delimiter `$function$`, it is just some
more characters within the constant so far as the outer string is
concerned.

The tag, if any, of a dollar-quoted string follows the same rules as an
unquoted identifier, except that it cannot contain a dollar sign. Tags
are case sensitive, so `$tag$String content$tag$` is correct, but
`$TAG$String content$tag$` is not.

A dollar-quoted string that follows a keyword or identifier must be
separated from it by whitespace; otherwise the dollar quoting delimiter
would be taken as part of the preceding identifier.

Dollar quoting is not part of the SQL standard, but it is often a more
convenient way to write complicated string literals than the
standard-compliant single quote syntax. It is particularly useful when
representing string constants inside other constants, as is often needed
in procedural function definitions. With single-quote syntax, each
backslash in the above example would have to be written as four
backslashes, which would be reduced to two backslashes in parsing the
original string constant, and then to one when the inner string constant
is re-parsed during function execution.

### Bit-String Constants {#sql-syntax-bit-strings}

bit string

constant

Bit-string constants look like regular string constants with a `B`
(upper or lower case) immediately before the opening quote (no
intervening whitespace), e.g., `B'1001'`. The only characters allowed
within bit-string constants are `0` and `1`.

Alternatively, bit-string constants can be specified in hexadecimal
notation, using a leading `X` (upper or lower case), e.g., `X'1FF'`.
This notation is equivalent to a bit-string constant with four binary
digits for each hexadecimal digit.

Both forms of bit-string constant can be continued across lines in the
same way as regular string constants. Dollar quoting cannot be used in a
bit-string constant.

### Numeric Constants {#sql-syntax-constants-numeric}

number

constant

Numeric constants are accepted in these general forms: digits
digits.\[digits\]\[e\[+-\]digits\] \[digits\].digits\[e\[+-\]digits\]
digitse\[+-\]digits where digits is one or more decimal digits (0
through 9). At least one digit must be before or after the decimal
point, if one is used. At least one digit must follow the exponent
marker (`e`), if one is present. There cannot be any spaces or other
characters embedded in the constant. Note that any leading plus or minus
sign is not actually considered part of the constant; it is an operator
applied to the constant.

These are some examples of valid numeric constants: 42 3.5 4. .001 5e2
1.925e-3

integer bigint numeric A numeric constant that contains neither a
decimal point nor an exponent is initially presumed to be type `integer`
if its value fits in type `integer` (32 bits); otherwise it is presumed
to be type `bigint` if its value fits in type `bigint` (64 bits);
otherwise it is taken to be type `numeric`. Constants that contain
decimal points and/or exponents are always initially presumed to be type
`numeric`.

The initially assigned data type of a numeric constant is just a
starting point for the type resolution algorithms. In most cases the
constant will be automatically coerced to the most appropriate type
depending on context. When necessary, you can force a numeric value to
be interpreted as a specific data type by casting it.type cast For
example, you can force a numeric value to be treated as type `real`
(`float4`) by writing:

    REAL '1.23'  -- string style
    1.23::REAL   -- PostgreSQL (historical) style

These are actually just special cases of the general casting notations
discussed next.

### Constants of Other Types {#sql-syntax-constants-generic}

data type

constant

A constant of an *arbitrary* type can be entered using any one of the
following notations: type \'string\' \'string\'::type CAST ( \'string\'
AS type ) The string constant\'s text is passed to the input conversion
routine for the type called type. The result is a constant of the
indicated type. The explicit type cast can be omitted if there is no
ambiguity as to the type the constant must be (for example, when it is
assigned directly to a table column), in which case it is automatically
coerced.

The string constant can be written using either regular SQL notation or
dollar-quoting.

It is also possible to specify a type coercion using a function-like
syntax: typename ( \'string\' ) but not all type names can be used in
this way; see [Type Casts](#sql-syntax-type-casts) for details.

The `::`, `CAST()`, and function-call syntaxes can also be used to
specify run-time type conversions of arbitrary expressions, as discussed
in [Type Casts](#sql-syntax-type-casts). To avoid syntactic ambiguity,
the `type 'string'` syntax can only be used to specify the type of a
simple literal constant. Another restriction on the `type 'string'`
syntax is that it does not work for array types; use `::` or `CAST()` to
specify the type of an array constant.

The `CAST()` syntax conforms to SQL. The `type 'string'` syntax is a
generalization of the standard: SQL specifies this syntax only for a few
data types, but PostgreSQL allows it for all types. The syntax with `::`
is historical PostgreSQL usage, as is the function-call syntax.

Operators {#sql-syntax-operators}
---------

operator

syntax

An operator name is a sequence of up to `NAMEDATALEN`-1 (63 by default)
characters from the following list: + - \* / \< \> = \~ ! @ \# % \^ & \|
\` ? There are a few restrictions on operator names, however:

-   `--` and `/*` cannot appear anywhere in an operator name, since they
    will be taken as the start of a comment.

-   A multiple-character operator name cannot end in `+` or `-`, unless
    the name also contains at least one of these characters: \~ ! @ \# %
    \^ & \| \` ? For example, `@-` is an allowed operator name, but `*-`
    is not. This restriction allows PostgreSQL to parse SQL-compliant
    queries without requiring spaces between tokens.

When working with non-SQL-standard operator names, you will usually need
to separate adjacent operators with spaces to avoid ambiguity. For
example, if you have defined a prefix operator named `@`, you cannot
write `X*@Y`; you must write `X* @Y` to ensure that PostgreSQL reads it
as two operator names not one.

Special Characters {#sql-syntax-special-chars}
------------------

Some characters that are not alphanumeric have a special meaning that is
different from being an operator. Details on the usage can be found at
the location where the respective syntax element is described. This
section only exists to advise the existence and summarize the purposes
of these characters.

-   A dollar sign (`$`) followed by digits is used to represent a
    positional parameter in the body of a function definition or a
    prepared statement. In other contexts the dollar sign can be part of
    an identifier or a dollar-quoted string constant.

-   Parentheses (`()`) have their usual meaning to group expressions and
    enforce precedence. In some cases parentheses are required as part
    of the fixed syntax of a particular SQL command.

-   Brackets (`[]`) are used to select the elements of an array. See
    [???](#arrays) for more information on arrays.

-   Commas (`,`) are used in some syntactical constructs to separate the
    elements of a list.

-   The semicolon (`;`) terminates an SQL command. It cannot appear
    anywhere within a command, except within a string constant or quoted
    identifier.

-   The colon (`:`) is used to select "slices" from arrays. (See
    [???](#arrays).) In certain SQL dialects (such as Embedded SQL), the
    colon is used to prefix variable names.

-   The asterisk (`*`) is used in some contexts to denote all the fields
    of a table row or composite value. It also has a special meaning
    when used as the argument of an aggregate function, namely that the
    aggregate does not require any explicit parameter.

-   The period (`.`) is used in numeric constants, and to separate
    schema, table, and column names.

Comments {#sql-syntax-comments}
--------

comment

in SQL

A comment is a sequence of characters beginning with double dashes and
extending to the end of the line, e.g.:

    -- This is a standard SQL comment

Alternatively, C-style block comments can be used:

    /* multiline comment
     * with nesting: /* nested block comment */
     */

where the comment begins with `/*` and extends to the matching
occurrence of `*/`. These block comments nest, as specified in the SQL
standard but unlike C, so that one can comment out larger blocks of code
that might contain existing block comments.

A comment is removed from the input stream before further syntax
analysis and is effectively replaced by whitespace.

Operator Precedence {#sql-precedence}
-------------------

operator

precedence

[table\_title](#sql-precedence-table) shows the precedence and
associativity of the operators in PostgreSQL. Most operators have the
same precedence and are left-associative. The precedence and
associativity of the operators is hard-wired into the parser. Add
parentheses if you want an expression with multiple operators to be
parsed in some other way than what the precedence rules imply.

  -------------------------------------------------------------------------
  Operator/Element             Associativity   Description
  ---------------------------- --------------- ----------------------------
  .                            left            table/column name separator

  ::                           left            PostgreSQL-style typecast

  \[ \]                        left            array element selection

  \+ -                         right           unary plus, unary minus

  \^                           left            exponentiation

  \* / %                       left            multiplication, division,
                                               modulo

  \+ -                         left            addition, subtraction

  (any other operator)         left            all other native and
                                               user-defined operators

  BETWEEN IN LIKE ILIKE                        range containment, set
  SIMILAR                                      membership, string matching

  \< \> = \<= \>= \<\>                         comparison operators

  IS ISNULL NOTNULL                            `IS TRUE`, `IS FALSE`, `IS
                                                      NULL`,
                                               `IS DISTINCT FROM`, etc

  NOT                          right           logical negation

  AND                          left            logical conjunction

  OR                           left            logical disjunction
  -------------------------------------------------------------------------

  : Operator Precedence (highest to lowest)

Note that the operator precedence rules also apply to user-defined
operators that have the same names as the built-in operators mentioned
above. For example, if you define a "+" operator for some custom data
type it will have the same precedence as the built-in "+" operator, no
matter what yours does.

When a schema-qualified operator name is used in the `OPERATOR` syntax,
as for example in:

    SELECT 3 OPERATOR(pg_catalog.+) 4;

the `OPERATOR` construct is taken to have the default precedence shown
in [table\_title](#sql-precedence-table) for "any other operator". This
is true no matter which specific operator appears inside `OPERATOR()`.

> **Note**
>
> PostgreSQL versions before 9.5 used slightly different operator
> precedence rules. In particular, \<= \>= and \<\> used to be treated
> as generic operators; `IS` tests used to have higher priority; and
> `NOT BETWEEN` and related constructs acted inconsistently, being taken
> in some cases as having the precedence of `NOT` rather than `BETWEEN`.
> These rules were changed for better compliance with the SQL standard
> and to reduce confusion from inconsistent treatment of logically
> equivalent constructs. In most cases, these changes will result in no
> behavioral change, or perhaps in "no such operator" failures which can
> be resolved by adding parentheses. However there are corner cases in
> which a query might change behavior without any parsing error being
> reported.

Value Expressions {#sql-expressions}
=================

expression

syntax

value expression

scalar

expression

Value expressions are used in a variety of contexts, such as in the
target list of the `SELECT` command, as new column values in `INSERT` or
`UPDATE`, or in search conditions in a number of commands. The result of
a value expression is sometimes called a scalar, to distinguish it from
the result of a table expression (which is a table). Value expressions
are therefore also called scalar expressions (or even simply
expressions). The expression syntax allows the calculation of values
from primitive parts using arithmetic, logical, set, and other
operations.

A value expression is one of the following:

-   A constant or literal value

-   A column reference

-   A positional parameter reference, in the body of a function
    definition or prepared statement

-   A subscripted expression

-   A field selection expression

-   An operator invocation

-   A function call

-   An aggregate expression

-   A window function call

-   A type cast

-   A collation expression

-   A scalar subquery

-   An array constructor

-   A row constructor

-   Another value expression in parentheses (used to group
    subexpressions and override precedenceparenthesis)

In addition to this list, there are a number of constructs that can be
classified as an expression but do not follow any general syntax rules.
These generally have the semantics of a function or operator and are
explained in the appropriate location in [???](#functions). An example
is the `IS NULL` clause.

We have already discussed constants in
[Constants](#sql-syntax-constants). The following sections discuss the
remaining options.

Column References {#sql-expressions-column-refs}
-----------------

column reference

A column can be referenced in the form: correlation.columnname

correlation is the name of a table (possibly qualified with a schema
name), or an alias for a table defined by means of a `FROM` clause. The
correlation name and separating dot can be omitted if the column name is
unique across all the tables being used in the current query. (See also
[???](#queries).)

Positional Parameters {#sql-expressions-parameters-positional}
---------------------

parameter

syntax

\$

A positional parameter reference is used to indicate a value that is
supplied externally to an SQL statement. Parameters are used in SQL
function definitions and in prepared queries. Some client libraries also
support specifying data values separately from the SQL command string,
in which case parameters are used to refer to the out-of-line data
values. The form of a parameter reference is: \$number

For example, consider the definition of a function, `dept`, as:

    CREATE FUNCTION dept(text) RETURNS dept
        AS $$ SELECT * FROM dept WHERE name = $1 $$
        LANGUAGE SQL;

Here the `$1` references the value of the first function argument
whenever the function is invoked.

Subscripts {#sql-expressions-subscripts}
----------

subscript

If an expression yields a value of an array type, then a specific
element of the array value can be extracted by writing
expression\[subscript\] or multiple adjacent elements (an "array slice")
can be extracted by writing
expression\[lower\_subscript:upper\_subscript\] (Here, the brackets
`[ ]` are meant to appear literally.) Each subscript is itself an
expression, which will be rounded to the nearest integer value.

In general the array expression must be parenthesized, but the
parentheses can be omitted when the expression to be subscripted is just
a column reference or positional parameter. Also, multiple subscripts
can be concatenated when the original array is multidimensional. For
example:

    mytable.arraycolumn[4]
    mytable.two_d_column[17][34]
    $1[10:42]
    (arrayfunction(a,b))[42]

The parentheses in the last example are required. See [???](#arrays) for
more about arrays.

Field Selection
---------------

field selection

If an expression yields a value of a composite type (row type), then a
specific field of the row can be extracted by writing
expression.fieldname

In general the row expression must be parenthesized, but the parentheses
can be omitted when the expression to be selected from is just a table
reference or positional parameter. For example:

    mytable.mycolumn
    $1.somecolumn
    (rowfunction(a,b)).col3

(Thus, a qualified column reference is actually just a special case of
the field selection syntax.) An important special case is extracting a
field from a table column that is of a composite type:

    (compositecol).somefield
    (mytable.compositecol).somefield

The parentheses are required here to show that compositecol is a column
name not a table name, or that mytable is a table name not a schema name
in the second case.

You can ask for all fields of a composite value by writing `.*`:

    (compositecol).*

This notation behaves differently depending on context; see
[???](#rowtypes-usage) for details.

Operator Invocations {#sql-expressions-operator-calls}
--------------------

operator

invocation

There are two possible syntaxes for an operator invocation: expression
operator expression (binary infix operator), operator expression (unary
prefix operator) where the operator token follows the syntax rules of
[Operators](#sql-syntax-operators), or is one of the key words AND, OR,
and NOT, or is a qualified operator name in the form:
`OPERATOR(`schema`.`operatorname`)` Which particular operators exist and
whether they are unary or binary depends on what operators have been
defined by the system or the user. [???](#functions) describes the
built-in operators.

Function Calls {#sql-expressions-function-calls}
--------------

function

invocation

The syntax for a function call is the name of a function (possibly
qualified with a schema name), followed by its argument list enclosed in
parentheses: function\_name (\[expression \[, expression \...\]\] )

For example, the following computes the square root of 2:

    sqrt(2)

The list of built-in functions is in [???](#functions). Other functions
can be added by the user.

When issuing queries in a database where some users mistrust other
users, observe security precautions from [???](#typeconv-func) when
writing function calls.

The arguments can optionally have names attached. See [Calling
Functions](#sql-syntax-calling-funcs) for details.

> **Note**
>
> A function that takes a single argument of composite type can
> optionally be called using field-selection syntax, and conversely
> field selection can be written in functional style. That is, the
> notations `col(table)` and `table.col` are interchangeable. This
> behavior is not SQL-standard but is provided in PostgreSQL because it
> allows use of functions to emulate "computed fields". For more
> information see [???](#rowtypes-usage).

Aggregate Expressions {#syntax-aggregates}
---------------------

aggregate function

invocation

ordered-set aggregate

WITHIN GROUP

FILTER

An aggregate expression represents the application of an aggregate
function across the rows selected by a query. An aggregate function
reduces multiple inputs to a single output value, such as the sum or
average of the inputs. The syntax of an aggregate expression is one of
the following: aggregate\_name (expression \[ , \... \] \[
order\_by\_clause \] ) \[ FILTER ( WHERE filter\_clause ) \]
aggregate\_name (ALL expression \[ , \... \] \[ order\_by\_clause \] )
\[ FILTER ( WHERE filter\_clause ) \] aggregate\_name (DISTINCT
expression \[ , \... \] \[ order\_by\_clause \] ) \[ FILTER ( WHERE
filter\_clause ) \] aggregate\_name ( \* ) \[ FILTER ( WHERE
filter\_clause ) \] aggregate\_name ( \[ expression \[ , \... \] \] )
WITHIN GROUP ( order\_by\_clause ) \[ FILTER ( WHERE filter\_clause ) \]
where aggregate\_name is a previously defined aggregate (possibly
qualified with a schema name) and expression is any value expression
that does not itself contain an aggregate expression or a window
function call. The optional order\_by\_clause and filter\_clause are
described below.

The first form of aggregate expression invokes the aggregate once for
each input row. The second form is the same as the first, since `ALL` is
the default. The third form invokes the aggregate once for each distinct
value of the expression (or distinct set of values, for multiple
expressions) found in the input rows. The fourth form invokes the
aggregate once for each input row; since no particular input value is
specified, it is generally only useful for the `count(*)` aggregate
function. The last form is used with ordered-set aggregate functions,
which are described below.

Most aggregate functions ignore null inputs, so that rows in which one
or more of the expression(s) yield null are discarded. This can be
assumed to be true, unless otherwise specified, for all built-in
aggregates.

For example, `count(*)` yields the total number of input rows;
`count(f1)` yields the number of input rows in which `f1` is non-null,
since `count` ignores nulls; and `count(distinct f1)` yields the number
of distinct non-null values of `f1`.

Ordinarily, the input rows are fed to the aggregate function in an
unspecified order. In many cases this does not matter; for example,
`min` produces the same result no matter what order it receives the
inputs in. However, some aggregate functions (such as `array_agg` and
`string_agg`) produce results that depend on the ordering of the input
rows. When using such an aggregate, the optional order\_by\_clause can
be used to specify the desired ordering. The order\_by\_clause has the
same syntax as for a query-level `ORDER BY` clause, as described in
[???](#queries-order), except that its expressions are always just
expressions and cannot be output-column names or numbers. For example:

    SELECT array_agg(a ORDER BY b DESC) FROM table;

When dealing with multiple-argument aggregate functions, note that the
`ORDER BY` clause goes after all the aggregate arguments. For example,
write this:

    SELECT string_agg(a, ',' ORDER BY a) FROM table;

not this:

    SELECT string_agg(a ORDER BY a, ',') FROM table;  -- incorrect

The latter is syntactically valid, but it represents a call of a
single-argument aggregate function with two `ORDER BY` keys (the second
one being rather useless since it\'s a constant).

If `DISTINCT` is specified in addition to an order\_by\_clause, then all
the `ORDER BY` expressions must match regular arguments of the
aggregate; that is, you cannot sort on an expression that is not
included in the `DISTINCT` list.

> **Note**
>
> The ability to specify both `DISTINCT` and `ORDER BY` in an aggregate
> function is a PostgreSQL extension.

Placing `ORDER BY` within the aggregate\'s regular argument list, as
described so far, is used when ordering the input rows for
general-purpose and statistical aggregates, for which ordering is
optional. There is a subclass of aggregate functions called ordered-set
aggregates for which an order\_by\_clause is *required*, usually because
the aggregate\'s computation is only sensible in terms of a specific
ordering of its input rows. Typical examples of ordered-set aggregates
include rank and percentile calculations. For an ordered-set aggregate,
the order\_by\_clause is written inside `WITHIN GROUP (...)`, as shown
in the final syntax alternative above. The expressions in the
order\_by\_clause are evaluated once per input row just like regular
aggregate arguments, sorted as per the order\_by\_clause\'s
requirements, and fed to the aggregate function as input arguments.
(This is unlike the case for a non-`WITHIN GROUP` order\_by\_clause,
which is not treated as argument(s) to the aggregate function.) The
argument expressions preceding `WITHIN GROUP`, if any, are called direct
arguments to distinguish them from the aggregated arguments listed in
the order\_by\_clause. Unlike regular aggregate arguments, direct
arguments are evaluated only once per aggregate call, not once per input
row. This means that they can contain variables only if those variables
are grouped by `GROUP BY`; this restriction is the same as if the direct
arguments were not inside an aggregate expression at all. Direct
arguments are typically used for things like percentile fractions, which
only make sense as a single value per aggregation calculation. The
direct argument list can be empty; in this case, write just `()` not
`(*)`. (PostgreSQL will actually accept either spelling, but only the
first way conforms to the SQL standard.)

median percentile An example of an ordered-set aggregate call is:

    SELECT percentile_cont(0.5) WITHIN GROUP (ORDER BY income) FROM households;
     percentile_cont
    -----------------
               50489

which obtains the 50th percentile, or median, value of the income column
from table households. Here, `0.5` is a direct argument; it would make
no sense for the percentile fraction to be a value varying across rows.

If `FILTER` is specified, then only the input rows for which the
filter\_clause evaluates to true are fed to the aggregate function;
other rows are discarded. For example:

    SELECT
        count(*) AS unfiltered,
        count(*) FILTER (WHERE i < 5) AS filtered
    FROM generate_series(1,10) AS s(i);
     unfiltered | filtered
    ------------+----------
             10 |        4
    (1 row)

The predefined aggregate functions are described in
[???](#functions-aggregate). Other aggregate functions can be added by
the user.

An aggregate expression can only appear in the result list or `HAVING`
clause of a `SELECT` command. It is forbidden in other clauses, such as
`WHERE`, because those clauses are logically evaluated before the
results of aggregates are formed.

When an aggregate expression appears in a subquery (see [Scalar
Subqueries](#sql-syntax-scalar-subqueries) and
[???](#functions-subquery)), the aggregate is normally evaluated over
the rows of the subquery. But an exception occurs if the aggregate\'s
arguments (and filter\_clause if any) contain only outer-level
variables: the aggregate then belongs to the nearest such outer level,
and is evaluated over the rows of that query. The aggregate expression
as a whole is then an outer reference for the subquery it appears in,
and acts as a constant over any one evaluation of that subquery. The
restriction about appearing only in the result list or `HAVING` clause
applies with respect to the query level that the aggregate belongs to.

Window Function Calls {#syntax-window-functions}
---------------------

window function

invocation

OVER clause

A window function call represents the application of an aggregate-like
function over some portion of the rows selected by a query. Unlike
non-window aggregate calls, this is not tied to grouping of the selected
rows into a single output row MDASH each row remains separate in the
query output. However the window function has access to all the rows
that would be part of the current row\'s group according to the grouping
specification (`PARTITION BY` list) of the window function call. The
syntax of a window function call is one of the following: function\_name
(\[expression \[, expression \...\]\]) \[ FILTER ( WHERE filter\_clause
) \] OVER window\_name function\_name (\[expression \[, expression
\...\]\]) \[ FILTER ( WHERE filter\_clause ) \] OVER (
window\_definition ) function\_name ( \* ) \[ FILTER ( WHERE
filter\_clause ) \] OVER window\_name function\_name ( \* ) \[ FILTER (
WHERE filter\_clause ) \] OVER ( window\_definition ) where
window\_definition has the syntax \[ existing\_window\_name \] \[
PARTITION BY expression \[, \...\] \] \[ ORDER BY expression \[ ASC \|
DESC \| USING operator \] \[ NULLS { FIRST \| LAST } \] \[, \...\] \] \[
frame\_clause \] The optional frame\_clause can be one of { RANGE \|
ROWS \| GROUPS } frame\_start \[ frame\_exclusion \] { RANGE \| ROWS \|
GROUPS } BETWEEN frame\_start AND frame\_end \[ frame\_exclusion \]
where frame\_start and frame\_end can be one of UNBOUNDED PRECEDING
offset PRECEDING CURRENT ROW offset FOLLOWING UNBOUNDED FOLLOWING and
frame\_exclusion can be one of EXCLUDE CURRENT ROW EXCLUDE GROUP EXCLUDE
TIES EXCLUDE NO OTHERS

Here, expression represents any value expression that does not itself
contain window function calls.

window\_name is a reference to a named window specification defined in
the query\'s `WINDOW` clause. Alternatively, a full window\_definition
can be given within parentheses, using the same syntax as for defining a
named window in the `WINDOW` clause; see the [???](#sql-select)
reference page for details. It\'s worth pointing out that `OVER wname`
is not exactly equivalent to `OVER (wname ...)`; the latter implies
copying and modifying the window definition, and will be rejected if the
referenced window specification includes a frame clause.

The `PARTITION BY` clause groups the rows of the query into partitions,
which are processed separately by the window function. `PARTITION BY`
works similarly to a query-level `GROUP BY` clause, except that its
expressions are always just expressions and cannot be output-column
names or numbers. Without `PARTITION BY`, all rows produced by the query
are treated as a single partition. The `ORDER BY` clause determines the
order in which the rows of a partition are processed by the window
function. It works similarly to a query-level `ORDER BY` clause, but
likewise cannot use output-column names or numbers. Without `ORDER BY`,
rows are processed in an unspecified order.

The frame\_clause specifies the set of rows constituting the window
frame, which is a subset of the current partition, for those window
functions that act on the frame instead of the whole partition. The set
of rows in the frame can vary depending on which row is the current row.
The frame can be specified in `RANGE`, `ROWS` or `GROUPS` mode; in each
case, it runs from the frame\_start to the frame\_end. If frame\_end is
omitted, the end defaults to `CURRENT ROW`.

A frame\_start of `UNBOUNDED PRECEDING` means that the frame starts with
the first row of the partition, and similarly a frame\_end of
`UNBOUNDED FOLLOWING` means that the frame ends with the last row of the
partition.

In `RANGE` or `GROUPS` mode, a frame\_start of `CURRENT ROW` means the
frame starts with the current row\'s first peer row (a row that the
window\'s `ORDER BY` clause sorts as equivalent to the current row),
while a frame\_end of `CURRENT ROW` means the frame ends with the
current row\'s last peer row. In `ROWS` mode, `CURRENT ROW` simply means
the current row.

In the offset `PRECEDING` and offset `FOLLOWING` frame options, the
offset must be an expression not containing any variables, aggregate
functions, or window functions. The meaning of the offset depends on the
frame mode:

-   In `ROWS` mode, the offset must yield a non-null, non-negative
    integer, and the option means that the frame starts or ends the
    specified number of rows before or after the current row.

-   In `GROUPS` mode, the offset again must yield a non-null,
    non-negative integer, and the option means that the frame starts or
    ends the specified number of peer groups before or after the current
    row\'s peer group, where a peer group is a set of rows that are
    equivalent in the `ORDER BY` ordering. (There must be an `ORDER BY`
    clause in the window definition to use `GROUPS` mode.)

-   In `RANGE` mode, these options require that the `ORDER BY` clause
    specify exactly one column. The offset specifies the maximum
    difference between the value of that column in the current row and
    its value in preceding or following rows of the frame. The data type
    of the offset expression varies depending on the data type of the
    ordering column. For numeric ordering columns it is typically of the
    same type as the ordering column, but for datetime ordering columns
    it is an `interval`. For example, if the ordering column is of type
    `date` or `timestamp`, one could write `RANGE BETWEEN
           '1 day' PRECEDING AND '10 days' FOLLOWING`. The offset is
    still required to be non-null and non-negative, though the meaning
    of "non-negative" depends on its data type.

In any case, the distance to the end of the frame is limited by the
distance to the end of the partition, so that for rows near the
partition ends the frame might contain fewer rows than elsewhere.

Notice that in both `ROWS` and `GROUPS` mode, `0 PRECEDING` and
`0 FOLLOWING` are equivalent to `CURRENT ROW`. This normally holds in
`RANGE` mode as well, for an appropriate data-type-specific meaning of
"zero".

The frame\_exclusion option allows rows around the current row to be
excluded from the frame, even if they would be included according to the
frame start and frame end options. `EXCLUDE CURRENT ROW` excludes the
current row from the frame. `EXCLUDE GROUP` excludes the current row and
its ordering peers from the frame. `EXCLUDE TIES` excludes any peers of
the current row from the frame, but not the current row itself.
`EXCLUDE NO OTHERS` simply specifies explicitly the default behavior of
not excluding the current row or its peers.

The default framing option is `RANGE UNBOUNDED PRECEDING`, which is the
same as `RANGE BETWEEN UNBOUNDED PRECEDING AND
    CURRENT ROW`. With `ORDER BY`, this sets the frame to be all rows
from the partition start up through the current row\'s last `ORDER BY`
peer. Without `ORDER BY`, this means all rows of the partition are
included in the window frame, since all rows become peers of the current
row.

Restrictions are that frame\_start cannot be `UNBOUNDED FOLLOWING`,
frame\_end cannot be `UNBOUNDED PRECEDING`, and the frame\_end choice
cannot appear earlier in the above list of frame\_start and frame\_end
options than the frame\_start choice does MDASH for example
`RANGE BETWEEN CURRENT ROW AND offset
    PRECEDING` is not allowed. But, for example,
`ROWS BETWEEN 7 PRECEDING AND 8
    PRECEDING` is allowed, even though it would never select any rows.

If `FILTER` is specified, then only the input rows for which the
filter\_clause evaluates to true are fed to the window function; other
rows are discarded. Only window functions that are aggregates accept a
`FILTER` clause.

The built-in window functions are described in
[???](#functions-window-table). Other window functions can be added by
the user. Also, any built-in or user-defined general-purpose or
statistical aggregate can be used as a window function. (Ordered-set and
hypothetical-set aggregates cannot presently be used as window
functions.)

The syntaxes using `*` are used for calling parameter-less aggregate
functions as window functions, for example
`count(*) OVER (PARTITION BY x ORDER BY y)`. The asterisk (`*`) is
customarily not used for window-specific functions. Window-specific
functions do not allow `DISTINCT` or `ORDER BY` to be used within the
function argument list.

Window function calls are permitted only in the `SELECT` list and the
`ORDER BY` clause of the query.

More information about window functions can be found in
[???](#tutorial-window), [???](#functions-window), and
[???](#queries-window).

Type Casts {#sql-syntax-type-casts}
----------

data type

type cast

type cast

::

A type cast specifies a conversion from one data type to another.
PostgreSQL accepts two equivalent syntaxes for type casts: CAST (
expression AS type ) expression::type The `CAST` syntax conforms to SQL;
the syntax with `::` is historical PostgreSQL usage.

When a cast is applied to a value expression of a known type, it
represents a run-time type conversion. The cast will succeed only if a
suitable type conversion operation has been defined. Notice that this is
subtly different from the use of casts with constants, as shown in
[Constants of Other Types](#sql-syntax-constants-generic). A cast
applied to an unadorned string literal represents the initial assignment
of a type to a literal constant value, and so it will succeed for any
type (if the contents of the string literal are acceptable input syntax
for the data type).

An explicit type cast can usually be omitted if there is no ambiguity as
to the type that a value expression must produce (for example, when it
is assigned to a table column); the system will automatically apply a
type cast in such cases. However, automatic casting is only done for
casts that are marked "OK to apply implicitly" in the system catalogs.
Other casts must be invoked with explicit casting syntax. This
restriction is intended to prevent surprising conversions from being
applied silently.

It is also possible to specify a type cast using a function-like syntax:
typename ( expression ) However, this only works for types whose names
are also valid as function names. For example, `double precision` cannot
be used this way, but the equivalent `float8` can. Also, the names
`interval`, `time`, and `timestamp` can only be used in this fashion if
they are double-quoted, because of syntactic conflicts. Therefore, the
use of the function-like cast syntax leads to inconsistencies and should
probably be avoided.

> **Note**
>
> The function-like syntax is in fact just a function call. When one of
> the two standard cast syntaxes is used to do a run-time conversion, it
> will internally invoke a registered function to perform the
> conversion. By convention, these conversion functions have the same
> name as their output type, and thus the "function-like syntax" is
> nothing more than a direct invocation of the underlying conversion
> function. Obviously, this is not something that a portable application
> should rely on. For further details see [???](#sql-createcast).

Collation Expressions {#sql-syntax-collate-exprs}
---------------------

COLLATE

The `COLLATE` clause overrides the collation of an expression. It is
appended to the expression it applies to: expr COLLATE collation where
collation is a possibly schema-qualified identifier. The `COLLATE`
clause binds tighter than operators; parentheses can be used when
necessary.

If no collation is explicitly specified, the database system either
derives a collation from the columns involved in the expression, or it
defaults to the default collation of the database if no column is
involved in the expression.

The two common uses of the `COLLATE` clause are overriding the sort
order in an `ORDER BY` clause, for example:

    SELECT a, b, c FROM tbl WHERE ... ORDER BY a COLLATE "C";

and overriding the collation of a function or operator call that has
locale-sensitive results, for example:

    SELECT * FROM tbl WHERE a > 'foo' COLLATE "C";

Note that in the latter case the `COLLATE` clause is attached to an
input argument of the operator we wish to affect. It doesn\'t matter
which argument of the operator or function call the `COLLATE` clause is
attached to, because the collation that is applied by the operator or
function is derived by considering all arguments, and an explicit
`COLLATE` clause will override the collations of all other arguments.
(Attaching non-matching `COLLATE` clauses to more than one argument,
however, is an error. For more details see [???](#collation).) Thus,
this gives the same result as the previous example:

    SELECT * FROM tbl WHERE a COLLATE "C" > 'foo';

But this is an error:

    SELECT * FROM tbl WHERE (a > 'foo') COLLATE "C";

because it attempts to apply a collation to the result of the `>`
operator, which is of the non-collatable data type `boolean`.

Scalar Subqueries {#sql-syntax-scalar-subqueries}
-----------------

subquery

A scalar subquery is an ordinary `SELECT` query in parentheses that
returns exactly one row with one column. (See [???](#queries) for
information about writing queries.) The `SELECT` query is executed and
the single returned value is used in the surrounding value expression.
It is an error to use a query that returns more than one row or more
than one column as a scalar subquery. (But if, during a particular
execution, the subquery returns no rows, there is no error; the scalar
result is taken to be null.) The subquery can refer to variables from
the surrounding query, which will act as constants during any one
evaluation of the subquery. See also [???](#functions-subquery) for
other expressions involving subqueries.

For example, the following finds the largest city population in each
state:

    SELECT name, (SELECT max(pop) FROM cities WHERE cities.state = states.name)
        FROM states;

Array Constructors {#sql-syntax-array-constructors}
------------------

array

constructor

ARRAY

An array constructor is an expression that builds an array value using
values for its member elements. A simple array constructor consists of
the key word `ARRAY`, a left square bracket `[`, a list of expressions
(separated by commas) for the array element values, and finally a right
square bracket `]`. For example:

    SELECT ARRAY[1,2,3+4];
      array
    ---------
     {1,2,7}
    (1 row)

By default, the array element type is the common type of the member
expressions, determined using the same rules as for `UNION` or `CASE`
constructs (see [???](#typeconv-union-case)). You can override this by
explicitly casting the array constructor to the desired type, for
example:

    SELECT ARRAY[1,2,22.7]::integer[];
      array
    ----------
     {1,2,23}
    (1 row)

This has the same effect as casting each expression to the array element
type individually. For more on casting, see [Type
Casts](#sql-syntax-type-casts).

Multidimensional array values can be built by nesting array
constructors. In the inner constructors, the key word `ARRAY` can be
omitted. For example, these produce the same result:

    SELECT ARRAY[ARRAY[1,2], ARRAY[3,4]];
         array
    ---------------
     {{1,2},{3,4}}
    (1 row)

    SELECT ARRAY[[1,2],[3,4]];
         array
    ---------------
     {{1,2},{3,4}}
    (1 row)

Since multidimensional arrays must be rectangular, inner constructors at
the same level must produce sub-arrays of identical dimensions. Any cast
applied to the outer `ARRAY` constructor propagates automatically to all
the inner constructors.

Multidimensional array constructor elements can be anything yielding an
array of the proper kind, not only a sub-`ARRAY` construct. For example:

    CREATE TABLE arr(f1 int[], f2 int[]);

    INSERT INTO arr VALUES (ARRAY[[1,2],[3,4]], ARRAY[[5,6],[7,8]]);

    SELECT ARRAY[f1, f2, '{{9,10},{11,12}}'::int[]] FROM arr;
                         array
    ------------------------------------------------
     {{{1,2},{3,4}},{{5,6},{7,8}},{{9,10},{11,12}}}
    (1 row)

You can construct an empty array, but since it\'s impossible to have an
array with no type, you must explicitly cast your empty array to the
desired type. For example:

    SELECT ARRAY[]::integer[];
     array
    -------
     {}
    (1 row)

It is also possible to construct an array from the results of a
subquery. In this form, the array constructor is written with the key
word `ARRAY` followed by a parenthesized (not bracketed) subquery. For
example:

    SELECT ARRAY(SELECT oid FROM pg_proc WHERE proname LIKE 'bytea%');
                                  array
    ------------------------------------------------------------------
     {2011,1954,1948,1952,1951,1244,1950,2005,1949,1953,2006,31,2412}
    (1 row)

    SELECT ARRAY(SELECT ARRAY[i, i*2] FROM generate_series(1,5) AS a(i));
                  array
    ----------------------------------
     {{1,2},{2,4},{3,6},{4,8},{5,10}}
    (1 row)

The subquery must return a single column. If the subquery\'s output
column is of a non-array type, the resulting one-dimensional array will
have an element for each row in the subquery result, with an element
type matching that of the subquery\'s output column. If the subquery\'s
output column is of an array type, the result will be an array of the
same type but one higher dimension; in this case all the subquery rows
must yield arrays of identical dimensionality, else the result would not
be rectangular.

The subscripts of an array value built with `ARRAY` always begin with
one. For more information about arrays, see [???](#arrays).

Row Constructors {#sql-syntax-row-constructors}
----------------

composite type

constructor

row type

constructor

ROW

A row constructor is an expression that builds a row value (also called
a composite value) using values for its member fields. A row constructor
consists of the key word `ROW`, a left parenthesis, zero or more
expressions (separated by commas) for the row field values, and finally
a right parenthesis. For example:

    SELECT ROW(1,2.5,'this is a test');

The key word `ROW` is optional when there is more than one expression in
the list.

A row constructor can include the syntax rowvalue`.*`, which will be
expanded to a list of the elements of the row value, just as occurs when
the `.*` syntax is used at the top level of a `SELECT` list (see
[???](#rowtypes-usage)). For example, if table `t` has columns `f1` and
`f2`, these are the same:

    SELECT ROW(t.*, 42) FROM t;
    SELECT ROW(t.f1, t.f2, 42) FROM t;

> **Note**
>
> Before PostgreSQL 8.2, the `.*` syntax was not expanded in row
> constructors, so that writing `ROW(t.*, 42)` created a two-field row
> whose first field was another row value. The new behavior is usually
> more useful. If you need the old behavior of nested row values, write
> the inner row value without `.*`, for instance `ROW(t, 42)`.

By default, the value created by a `ROW` expression is of an anonymous
record type. If necessary, it can be cast to a named composite type
MDASH either the row type of a table, or a composite type created with
`CREATE TYPE AS`. An explicit cast might be needed to avoid ambiguity.
For example:

    CREATE TABLE mytable(f1 int, f2 float, f3 text);

    CREATE FUNCTION getf1(mytable) RETURNS int AS 'SELECT $1.f1' LANGUAGE SQL;

    -- No cast needed since only one getf1() exists
    SELECT getf1(ROW(1,2.5,'this is a test'));
     getf1
    -------
         1
    (1 row)

    CREATE TYPE myrowtype AS (f1 int, f2 text, f3 numeric);

    CREATE FUNCTION getf1(myrowtype) RETURNS int AS 'SELECT $1.f1' LANGUAGE SQL;

    -- Now we need a cast to indicate which function to call:
    SELECT getf1(ROW(1,2.5,'this is a test'));
    ERROR:  function getf1(record) is not unique

    SELECT getf1(ROW(1,2.5,'this is a test')::mytable);
     getf1
    -------
         1
    (1 row)

    SELECT getf1(CAST(ROW(11,'this is a test',2.5) AS myrowtype));
     getf1
    -------
        11
    (1 row)

Row constructors can be used to build composite values to be stored in a
composite-type table column, or to be passed to a function that accepts
a composite parameter. Also, it is possible to compare two row values or
test a row with `IS NULL` or `IS NOT NULL`, for example:

    SELECT ROW(1,2.5,'this is a test') = ROW(1, 3, 'not the same');

    SELECT ROW(table.*) IS NULL FROM table;  -- detect all-null rows

For more detail see [???](#functions-comparisons). Row constructors can
also be used in connection with subqueries, as discussed in
[???](#functions-subquery).

Expression Evaluation Rules {#syntax-express-eval}
---------------------------

expression

order of evaluation

The order of evaluation of subexpressions is not defined. In particular,
the inputs of an operator or function are not necessarily evaluated
left-to-right or in any other fixed order.

Furthermore, if the result of an expression can be determined by
evaluating only some parts of it, then other subexpressions might not be
evaluated at all. For instance, if one wrote:

    SELECT true OR somefunc();

then `somefunc()` would (probably) not be called at all. The same would
be the case if one wrote:

    SELECT somefunc() OR true;

Note that this is not the same as the left-to-right "short-circuiting"
of Boolean operators that is found in some programming languages.

As a consequence, it is unwise to use functions with side effects as
part of complex expressions. It is particularly dangerous to rely on
side effects or evaluation order in `WHERE` and `HAVING` clauses, since
those clauses are extensively reprocessed as part of developing an
execution plan. Boolean expressions (`AND`/`OR`/`NOT` combinations) in
those clauses can be reorganized in any manner allowed by the laws of
Boolean algebra.

When it is essential to force evaluation order, a `CASE` construct (see
[???](#functions-conditional)) can be used. For example, this is an
untrustworthy way of trying to avoid division by zero in a `WHERE`
clause:

    SELECT ... WHERE x > 0 AND y/x > 1.5;

But this is safe:

    SELECT ... WHERE CASE WHEN x > 0 THEN y/x > 1.5 ELSE false END;

A `CASE` construct used in this fashion will defeat optimization
attempts, so it should only be done when necessary. (In this particular
example, it would be better to sidestep the problem by writing
`y > 1.5*x` instead.)

`CASE` is not a cure-all for such issues, however. One limitation of the
technique illustrated above is that it does not prevent early evaluation
of constant subexpressions. As described in [???](#xfunc-volatility),
functions and operators marked `IMMUTABLE` can be evaluated when the
query is planned rather than when it is executed. Thus for example

    SELECT CASE WHEN x > 0 THEN x ELSE 1/0 END FROM tab;

is likely to result in a division-by-zero failure due to the planner
trying to simplify the constant subexpression, even if every row in the
table has `x > 0` so that the `ELSE` arm would never be entered at run
time.

While that particular example might seem silly, related cases that
don\'t obviously involve constants can occur in queries executed within
functions, since the values of function arguments and local variables
can be inserted into queries as constants for planning purposes. Within
PL/pgSQL functions, for example, using an `IF`-`THEN`-`ELSE` statement
to protect a risky computation is much safer than just nesting it in a
`CASE` expression.

Another limitation of the same kind is that a `CASE` cannot prevent
evaluation of an aggregate expression contained within it, because
aggregate expressions are computed before other expressions in a
`SELECT` list or `HAVING` clause are considered. For example, the
following query can cause a division-by-zero error despite seemingly
having protected against it:

    SELECT CASE WHEN min(employees) > 0
                THEN avg(expenses / employees)
           END
        FROM departments;

The `min()` and `avg()` aggregates are computed concurrently over all
the input rows, so if any row has employees equal to zero, the
division-by-zero error will occur before there is any opportunity to
test the result of `min()`. Instead, use a `WHERE` or `FILTER` clause to
prevent problematic input rows from reaching an aggregate function in
the first place.

Calling Functions {#sql-syntax-calling-funcs}
=================

notation

functions

PostgreSQL allows functions that have named parameters to be called
using either positional or named notation. Named notation is especially
useful for functions that have a large number of parameters, since it
makes the associations between parameters and actual arguments more
explicit and reliable. In positional notation, a function call is
written with its argument values in the same order as they are defined
in the function declaration. In named notation, the arguments are
matched to the function parameters by name and can be written in any
order. For each notation, also consider the effect of function argument
types, documented in [???](#typeconv-func).

In either notation, parameters that have default values given in the
function declaration need not be written in the call at all. But this is
particularly useful in named notation, since any combination of
parameters can be omitted; while in positional notation parameters can
only be omitted from right to left.

PostgreSQL also supports mixed notation, which combines positional and
named notation. In this case, positional parameters are written first
and named parameters appear after them.

The following examples will illustrate the usage of all three notations,
using the following function definition:

    CREATE FUNCTION concat_lower_or_upper(a text, b text, uppercase boolean DEFAULT false)
    RETURNS text
    AS
    $$
     SELECT CASE
            WHEN $3 THEN UPPER($1 || ' ' || $2)
            ELSE LOWER($1 || ' ' || $2)
            END;
    $$
    LANGUAGE SQL IMMUTABLE STRICT;

Function `concat_lower_or_upper` has two mandatory parameters, `a` and
`b`. Additionally there is one optional parameter `uppercase` which
defaults to `false`. The `a` and `b` inputs will be concatenated, and
forced to either upper or lower case depending on the `uppercase`
parameter. The remaining details of this function definition are not
important here (see [???](#extend) for more information).

Using Positional Notation {#sql-syntax-calling-funcs-positional}
-------------------------

function

positional notation

Positional notation is the traditional mechanism for passing arguments
to functions in PostgreSQL. An example is:

    SELECT concat_lower_or_upper('Hello', 'World', true);
     concat_lower_or_upper 
    -----------------------
     HELLO WORLD
    (1 row)

All arguments are specified in order. The result is upper case since
`uppercase` is specified as `true`. Another example is:

    SELECT concat_lower_or_upper('Hello', 'World');
     concat_lower_or_upper 
    -----------------------
     hello world
    (1 row)

Here, the `uppercase` parameter is omitted, so it receives its default
value of `false`, resulting in lower case output. In positional
notation, arguments can be omitted from right to left so long as they
have defaults.

Using Named Notation {#sql-syntax-calling-funcs-named}
--------------------

function

named notation

In named notation, each argument\'s name is specified using `=>` to
separate it from the argument expression. For example:

    SELECT concat_lower_or_upper(a => 'Hello', b => 'World');
     concat_lower_or_upper 
    -----------------------
     hello world
    (1 row)

Again, the argument `uppercase` was omitted so it is set to `false`
implicitly. One advantage of using named notation is that the arguments
may be specified in any order, for example:

    SELECT concat_lower_or_upper(a => 'Hello', b => 'World', uppercase => true);
     concat_lower_or_upper 
    -----------------------
     HELLO WORLD
    (1 row)

    SELECT concat_lower_or_upper(a => 'Hello', uppercase => true, b => 'World');
     concat_lower_or_upper 
    -----------------------
     HELLO WORLD
    (1 row)

An older syntax based on \":=\" is supported for backward compatibility:

    SELECT concat_lower_or_upper(a := 'Hello', uppercase := true, b := 'World');
     concat_lower_or_upper 
    -----------------------
     HELLO WORLD
    (1 row)

Using Mixed Notation {#sql-syntax-calling-funcs-mixed}
--------------------

function

mixed notation

The mixed notation combines positional and named notation. However, as
already mentioned, named arguments cannot precede positional arguments.
For example:

    SELECT concat_lower_or_upper('Hello', 'World', uppercase => true);
     concat_lower_or_upper 
    -----------------------
     HELLO WORLD
    (1 row)

In the above query, the arguments `a` and `b` are specified
positionally, while `uppercase` is specified by name. In this example,
that adds little except documentation. With a more complex function
having numerous parameters that have default values, named or mixed
notation can save a great deal of writing and reduce chances for error.

> **Note**
>
> Named and mixed call notations currently cannot be used when calling
> an aggregate function (but they do work when an aggregate function is
> used as a window function).
