SELECT

TABLE command

WITH

in SELECT

SELECT

7

SQL - Language Statements

SELECT

TABLE

WITH

retrieve rows from a table or view

\[ WITH \[ RECURSIVE \]

with\_query

\[, \...\] \] SELECT \[ ALL \| DISTINCT \[ ON (

expression

\[, \...\] ) \] \] \[ \* \|

expression

\[ \[ AS \]

output\_name

\] \[, \...\] \] \[ FROM

from\_item

\[, \...\] \] \[ WHERE

condition

\] \[ GROUP BY \[ ALL \| DISTINCT \]

grouping\_element

\[, \...\] \] \[ HAVING

condition

\] \[ WINDOW

window\_name

AS (

window\_definition

) \[, \...\] \] \[ { UNION \| INTERSECT \| EXCEPT } \[ ALL \| DISTINCT
\]

select

\] \[ ORDER BY

expression

\[ ASC \| DESC \| USING

operator

\] \[ NULLS { FIRST \| LAST } \] \[, \...\] \] \[ LIMIT {

count

\| ALL } \] \[ OFFSET

start

\[ ROW \| ROWS \] \] \[ FETCH { FIRST \| NEXT } \[

count

\] { ROW \| ROWS } { ONLY \| WITH TIES } \] \[ FOR { UPDATE \| NO KEY
UPDATE \| SHARE \| KEY SHARE } \[ OF

table\_name

\[, \...\] \] \[ NOWAIT \| SKIP LOCKED \] \[\...\] \]

where

from\_item

can be one of:

\[ ONLY \]

table\_name

\[ \* \] \[ \[ AS \]

alias

\[ (

column\_alias

\[, \...\] ) \] \] \[ TABLESAMPLE

sampling\_method

(

argument

\[, \...\] ) \[ REPEATABLE (

seed

) \] \] \[ LATERAL \] (

select

) \[ AS \]

alias

\[ (

column\_alias

\[, \...\] ) \]

with\_query\_name

\[ \[ AS \]

alias

\[ (

column\_alias

\[, \...\] ) \] \] \[ LATERAL \]

function\_name

( \[

argument

\[, \...\] \] ) \[ WITH ORDINALITY \] \[ \[ AS \]

alias

\[ (

column\_alias

\[, \...\] ) \] \] \[ LATERAL \]

function\_name

( \[

argument

\[, \...\] \] ) \[ AS \]

alias

(

column\_definition

\[, \...\] ) \[ LATERAL \]

function\_name

( \[

argument

\[, \...\] \] ) AS (

column\_definition

\[, \...\] ) \[ LATERAL \] ROWS FROM(

function\_name

( \[

argument

\[, \...\] \] ) \[ AS (

column\_definition

\[, \...\] ) \] \[, \...\] ) \[ WITH ORDINALITY \] \[ \[ AS \]

alias

\[ (

column\_alias

\[, \...\] ) \] \]

from\_item

\[ NATURAL \]

join\_type

from\_item

\[ ON

join\_condition

\| USING (

join\_column

\[, \...\] ) \[ AS

join\_using\_alias

\] \]

and

grouping\_element

can be one of:

( )

expression

(

expression

\[, \...\] ) ROLLUP ( {

expression

\| (

expression

\[, \...\] ) } \[, \...\] ) CUBE ( {

expression

\| (

expression

\[, \...\] ) } \[, \...\] ) GROUPING SETS (

grouping\_element

\[, \...\] )

and

with\_query

is:

with\_query\_name

\[ (

column\_name

\[, \...\] ) \] AS \[ \[ NOT \] MATERIALIZED \] (

select

\|

values

\|

insert

\|

update

\|

delete

) \[ SEARCH { BREADTH \| DEPTH } FIRST BY

column\_name

\[, \...\] SET

search\_seq\_col\_name

\] \[ CYCLE

column\_name

\[, \...\] SET

cycle\_mark\_col\_name

\[ TO

cycle\_mark\_value

DEFAULT

cycle\_mark\_default

\] USING

cycle\_path\_col\_name

\] TABLE \[ ONLY \]

table\_name

\[ \* \]

Description
===========

`SELECT` retrieves rows from zero or more tables. The general processing
of `SELECT` is as follows:

1.  All queries in the `WITH` list are computed. These effectively serve
    as temporary tables that can be referenced in the `FROM` list. A
    `WITH` query that is referenced more than once in `FROM` is computed
    only once, unless specified otherwise with `NOT MATERIALIZED`. (See
    [WITH Clause](#sql-with) below.)

2.  All elements in the `FROM` list are computed. (Each element in the
    `FROM` list is a real or virtual table.) If more than one element is
    specified in the `FROM` list, they are cross-joined together. (See
    [FROM Clause](#sql-from) below.)

3.  If the `WHERE` clause is specified, all rows that do not satisfy the
    condition are eliminated from the output. (See [WHERE
    Clause](#sql-where) below.)

4.  If the `GROUP BY` clause is specified, or if there are aggregate
    function calls, the output is combined into groups of rows that
    match on one or more values, and the results of aggregate functions
    are computed. If the `HAVING` clause is present, it eliminates
    groups that do not satisfy the given condition. (See [GROUP BY
    Clause](#sql-groupby) and [HAVING Clause](#sql-having) below.)

5.  The actual output rows are computed using the `SELECT` output
    expressions for each selected row or row group. (See [SELECT
    List](#sql-select-list) below.)

6.  `SELECT DISTINCT` eliminates duplicate rows from the result.
    `SELECT DISTINCT ON` eliminates rows that match on all the specified
    expressions. `SELECT ALL` (the default) will return all candidate
    rows, including duplicates. (See [DISTINCT Clause](#sql-distinct)
    below.)

7.  Using the operators `UNION`, `INTERSECT`, and `EXCEPT`, the output
    of more than one `SELECT` statement can be combined to form a single
    result set. The `UNION` operator returns all rows that are in one or
    both of the result sets. The `INTERSECT` operator returns all rows
    that are strictly in both result sets. The `EXCEPT` operator returns
    the rows that are in the first result set but not in the second. In
    all three cases, duplicate rows are eliminated unless `ALL` is
    specified. The noise word `DISTINCT` can be added to explicitly
    specify eliminating duplicate rows. Notice that `DISTINCT` is the
    default behavior here, even though `ALL` is the default for `SELECT`
    itself. (See [UNION Clause](#sql-union), [INTERSECT
    Clause](#sql-intersect), and [EXCEPT Clause](#sql-except) below.)

8.  If the `ORDER BY` clause is specified, the returned rows are sorted
    in the specified order. If `ORDER BY` is not given, the rows are
    returned in whatever order the system finds fastest to produce. (See
    [ORDER BY Clause](#sql-orderby) below.)

9.  If the `LIMIT` (or `FETCH FIRST`) or `OFFSET` clause is specified,
    the `SELECT` statement only returns a subset of the result rows.
    (See [LIMIT Clause](#sql-limit) below.)

10. If `FOR UPDATE`, `FOR NO KEY UPDATE`, `FOR SHARE` or `FOR KEY SHARE`
    is specified, the `SELECT` statement locks the selected rows against
    concurrent updates. (See [The Locking Clause](#sql-for-update-share)
    below.)

You must have `SELECT` privilege on each column used in a `SELECT`
command. The use of `FOR NO KEY UPDATE`, `FOR UPDATE`, `FOR SHARE` or
`FOR KEY SHARE` requires `UPDATE` privilege as well (for at least one
column of each table so selected).

Parameters
==========

`WITH` Clause {#sql-with}
-------------

The `WITH` clause allows you to specify one or more subqueries that can
be referenced by name in the primary query. The subqueries effectively
act as temporary tables or views for the duration of the primary query.
Each subquery can be a `SELECT`, `TABLE`, `VALUES`, `INSERT`, `UPDATE`
or `DELETE` statement. When writing a data-modifying statement
(`INSERT`, `UPDATE` or `DELETE`) in `WITH`, it is usual to include a
`RETURNING` clause. It is the output of `RETURNING`, *not* the
underlying table that the statement modifies, that forms the temporary
table that is read by the primary query. If `RETURNING` is omitted, the
statement is still executed, but it produces no output so it cannot be
referenced as a table by the primary query.

A name (without schema qualification) must be specified for each `WITH`
query. Optionally, a list of column names can be specified; if this is
omitted, the column names are inferred from the subquery.

If `RECURSIVE` is specified, it allows a `SELECT` subquery to reference
itself by name. Such a subquery must have the form non\_recursive\_term
UNION \[ ALL \| DISTINCT \] recursive\_term where the recursive
self-reference must appear on the right-hand side of the `UNION`. Only
one recursive self-reference is permitted per query. Recursive
data-modifying statements are not supported, but you can use the results
of a recursive `SELECT` query in a data-modifying statement. See
[???](#queries-with) for an example.

Another effect of `RECURSIVE` is that `WITH` queries need not be
ordered: a query can reference another one that is later in the list.
(However, circular references, or mutual recursion, are not
implemented.) Without `RECURSIVE`, `WITH` queries can only reference
sibling `WITH` queries that are earlier in the `WITH` list.

When there are multiple queries in the `WITH` clause, `RECURSIVE` should
be written only once, immediately after `WITH`. It applies to all
queries in the `WITH` clause, though it has no effect on queries that do
not use recursion or forward references.

The optional `SEARCH` clause computes a search sequence column that can
be used for ordering the results of a recursive query in either
breadth-first or depth-first order. The supplied column name list
specifies the row key that is to be used for keeping track of visited
rows. A column named search\_seq\_col\_name will be added to the result
column list of the `WITH` query. This column can be ordered by in the
outer query to achieve the respective ordering. See
[???](#queries-with-search) for examples.

The optional `CYCLE` clause is used to detect cycles in recursive
queries. The supplied column name list specifies the row key that is to
be used for keeping track of visited rows. A column named
cycle\_mark\_col\_name will be added to the result column list of the
`WITH` query. This column will be set to cycle\_mark\_value when a cycle
has been detected, else to cycle\_mark\_default. Furthermore, processing
of the recursive union will stop when a cycle has been detected.
cycle\_mark\_value and cycle\_mark\_default must be constants and they
must be coercible to a common data type, and the data type must have an
inequality operator. (The SQL standard requires that they be Boolean
constants or character strings, but PostgreSQL does not require that.)
By default, `TRUE` and `FALSE` (of type `boolean`) are used.
Furthermore, a column named cycle\_path\_col\_name will be added to the
result column list of the `WITH` query. This column is used internally
for tracking visited rows. See [???](#queries-with-cycle) for examples.

Both the `SEARCH` and the `CYCLE` clause are only valid for recursive
`WITH` queries. The with\_query must be a `UNION` (or `UNION ALL`) of
two `SELECT` (or equivalent) commands (no nested `UNION`s). If both
clauses are used, the column added by the `SEARCH` clause appears before
the columns added by the `CYCLE` clause.

The primary query and the `WITH` queries are all (notionally) executed
at the same time. This implies that the effects of a data-modifying
statement in `WITH` cannot be seen from other parts of the query, other
than by reading its `RETURNING` output. If two such data-modifying
statements attempt to modify the same row, the results are unspecified.

A key property of `WITH` queries is that they are normally evaluated
only once per execution of the primary query, even if the primary query
refers to them more than once. In particular, data-modifying statements
are guaranteed to be executed once and only once, regardless of whether
the primary query reads all or any of their output.

However, a `WITH` query can be marked `NOT MATERIALIZED` to remove this
guarantee. In that case, the `WITH` query can be folded into the primary
query much as though it were a simple sub-`SELECT` in the primary
query\'s `FROM` clause. This results in duplicate computations if the
primary query refers to that `WITH` query more than once; but if each
such use requires only a few rows of the `WITH` query\'s total output,
`NOT MATERIALIZED` can provide a net savings by allowing the queries to
be optimized jointly. `NOT MATERIALIZED` is ignored if it is attached to
a `WITH` query that is recursive or is not side-effect-free (i.e., is
not a plain `SELECT` containing no volatile functions).

By default, a side-effect-free `WITH` query is folded into the primary
query if it is used exactly once in the primary query\'s `FROM` clause.
This allows joint optimization of the two query levels in situations
where that should be semantically invisible. However, such folding can
be prevented by marking the `WITH` query as `MATERIALIZED`. That might
be useful, for example, if the `WITH` query is being used as an
optimization fence to prevent the planner from choosing a bad plan.
PostgreSQL versions before v12 never did such folding, so queries
written for older versions might rely on `WITH` to act as an
optimization fence.

See [???](#queries-with) for additional information.

`FROM` Clause {#sql-from}
-------------

The `FROM` clause specifies one or more source tables for the `SELECT`.
If multiple sources are specified, the result is the Cartesian product
(cross join) of all the sources. But usually qualification conditions
are added (via `WHERE`) to restrict the returned rows to a small subset
of the Cartesian product.

The `FROM` clause can contain the following elements:

table\_name

:   The name (optionally schema-qualified) of an existing table or view.
    If `ONLY` is specified before the table name, only that table is
    scanned. If `ONLY` is not specified, the table and all its
    descendant tables (if any) are scanned. Optionally, `*` can be
    specified after the table name to explicitly indicate that
    descendant tables are included.

alias

:   A substitute name for the `FROM` item containing the alias. An alias
    is used for brevity or to eliminate ambiguity for self-joins (where
    the same table is scanned multiple times). When an alias is
    provided, it completely hides the actual name of the table or
    function; for example given `FROM foo AS f`, the remainder of the
    `SELECT` must refer to this `FROM` item as `f` not `foo`. If an
    alias is written, a column alias list can also be written to provide
    substitute names for one or more columns of the table.

`TABLESAMPLE sampling_method ( argument [, ...] ) [ REPEATABLE ( seed ) ]`

:   A `TABLESAMPLE` clause after a table\_name indicates that the
    specified sampling\_method should be used to retrieve a subset of
    the rows in that table. This sampling precedes the application of
    any other filters such as `WHERE` clauses. The standard PostgreSQL
    distribution includes two sampling methods, `BERNOULLI` and
    `SYSTEM`, and other sampling methods can be installed in the
    database via extensions.

    The `BERNOULLI` and `SYSTEM` sampling methods each accept a single
    argument which is the fraction of the table to sample, expressed as
    a percentage between 0 and 100. This argument can be any
    `real`-valued expression. (Other sampling methods might accept more
    or different arguments.) These two methods each return a
    randomly-chosen sample of the table that will contain approximately
    the specified percentage of the table\'s rows. The `BERNOULLI`
    method scans the whole table and selects or ignores individual rows
    independently with the specified probability. The `SYSTEM` method
    does block-level sampling with each block having the specified
    chance of being selected; all rows in each selected block are
    returned. The `SYSTEM` method is significantly faster than the
    `BERNOULLI` method when small sampling percentages are specified,
    but it may return a less-random sample of the table as a result of
    clustering effects.

    The optional `REPEATABLE` clause specifies a seed number or
    expression to use for generating random numbers within the sampling
    method. The seed value can be any non-null floating-point value. Two
    queries that specify the same seed and argument values will select
    the same sample of the table, if the table has not been changed
    meanwhile. But different seed values will usually produce different
    samples. If `REPEATABLE` is not given then a new random sample is
    selected for each query, based upon a system-generated seed. Note
    that some add-on sampling methods do not accept `REPEATABLE`, and
    will always produce new samples on each use.

select

:   A sub-`SELECT` can appear in the `FROM` clause. This acts as though
    its output were created as a temporary table for the duration of
    this single `SELECT` command. Note that the sub-`SELECT` must be
    surrounded by parentheses, and an alias *must* be provided for it. A
    [`VALUES`](#sql-values) command can also be used here.

with\_query\_name

:   A `WITH` query is referenced by writing its name, just as though the
    query\'s name were a table name. (In fact, the `WITH` query hides
    any real table of the same name for the purposes of the primary
    query. If necessary, you can refer to a real table of the same name
    by schema-qualifying the table\'s name.) An alias can be provided in
    the same way as for a table.

function\_name

:   Function calls can appear in the `FROM` clause. (This is especially
    useful for functions that return result sets, but any function can
    be used.) This acts as though the function\'s output were created as
    a temporary table for the duration of this single `SELECT` command.
    If the function\'s result type is composite (including the case of a
    function with multiple `OUT` parameters), each attribute becomes a
    separate column in the implicit table.

    When the optional `WITH ORDINALITY` clause is added to the function
    call, an additional column of type `bigint` will be appended to the
    function\'s result column(s). This column numbers the rows of the
    function\'s result set, starting from 1. By default, this column is
    named `ordinality`.

    An alias can be provided in the same way as for a table. If an alias
    is written, a column alias list can also be written to provide
    substitute names for one or more attributes of the function\'s
    composite return type, including the ordinality column if present.

    Multiple function calls can be combined into a single `FROM`-clause
    item by surrounding them with `ROWS FROM( ... )`. The output of such
    an item is the concatenation of the first row from each function,
    then the second row from each function, etc. If some of the
    functions produce fewer rows than others, null values are
    substituted for the missing data, so that the total number of rows
    returned is always the same as for the function that produced the
    most rows.

    If the function has been defined as returning the `record` data
    type, then an alias or the key word `AS` must be present, followed
    by a column definition list in the form
    `( column_name data_type , ...
            )`. The column definition list must match the actual number
    and types of columns returned by the function.

    When using the `ROWS FROM( ... )` syntax, if one of the functions
    requires a column definition list, it\'s preferred to put the column
    definition list after the function call inside `ROWS FROM( ... )`. A
    column definition list can be placed after the `ROWS FROM( ... )`
    construct only if there\'s just a single function and no
    `WITH ORDINALITY` clause.

    To use `ORDINALITY` together with a column definition list, you must
    use the `ROWS FROM( ... )` syntax and put the column definition list
    inside `ROWS FROM( ... )`.

join\_type

:   One of

    -   `[ INNER ] JOIN`

    -   `LEFT [ OUTER ] JOIN`

    -   `RIGHT [ OUTER ] JOIN`

    -   `FULL [ OUTER ] JOIN`

    -   `CROSS JOIN`

    For the `INNER` and `OUTER` join types, a join condition must be
    specified, namely exactly one of `NATURAL`, `ON join_condition`, or
    `USING (join_column [, ...])`. See below for the meaning. For
    `CROSS JOIN`, none of these clauses can appear.

    A `JOIN` clause combines two `FROM` items, which for convenience we
    will refer to as "tables", though in reality they can be any type of
    `FROM` item. Use parentheses if necessary to determine the order of
    nesting. In the absence of parentheses, `JOIN`s nest left-to-right.
    In any case `JOIN` binds more tightly than the commas separating
    `FROM`-list items.

    `CROSS JOIN` and `INNER JOIN` produce a simple Cartesian product,
    the same result as you get from listing the two tables at the top
    level of `FROM`, but restricted by the join condition (if any).
    `CROSS JOIN` is equivalent to `INNER JOIN ON
            (TRUE)`, that is, no rows are removed by qualification.
    These join types are just a notational convenience, since they do
    nothing you couldn\'t do with plain `FROM` and `WHERE`.

    `LEFT OUTER JOIN` returns all rows in the qualified Cartesian
    product (i.e., all combined rows that pass its join condition), plus
    one copy of each row in the left-hand table for which there was no
    right-hand row that passed the join condition. This left-hand row is
    extended to the full width of the joined table by inserting null
    values for the right-hand columns. Note that only the `JOIN`
    clause\'s own condition is considered while deciding which rows have
    matches. Outer conditions are applied afterwards.

    Conversely, `RIGHT OUTER JOIN` returns all the joined rows, plus one
    row for each unmatched right-hand row (extended with nulls on the
    left). This is just a notational convenience, since you could
    convert it to a `LEFT
            OUTER JOIN` by switching the left and right tables.

    `FULL OUTER JOIN` returns all the joined rows, plus one row for each
    unmatched left-hand row (extended with nulls on the right), plus one
    row for each unmatched right-hand row (extended with nulls on the
    left).

`ON join_condition`

:   join\_condition is an expression resulting in a value of type
    `boolean` (similar to a `WHERE` clause) that specifies which rows in
    a join are considered to match.

`USING ( join_column [, ...] ) [ AS join_using_alias ]`

:   A clause of the form `USING ( a, b, ... )` is shorthand for
    `ON left_table.a = right_table.a AND
            left_table.b = right_table.b ...`. Also, `USING` implies
    that only one of each pair of equivalent columns will be included in
    the join output, not both.

    If a join\_using\_alias name is specified, it provides a table alias
    for the join columns. Only the join columns listed in the `USING`
    clause are addressable by this name. Unlike a regular alias, this
    does not hide the names of the joined tables from the rest of the
    query. Also unlike a regular alias, you cannot write a column alias
    list MDASH the output names of the join columns are the same as they
    appear in the `USING` list.

`NATURAL`

:   `NATURAL` is shorthand for a `USING` list that mentions all columns
    in the two tables that have matching names. If there are no common
    column names, `NATURAL` is equivalent to `ON TRUE`.

`LATERAL`

:   The `LATERAL` key word can precede a sub-`SELECT` `FROM` item. This
    allows the sub-`SELECT` to refer to columns of `FROM` items that
    appear before it in the `FROM` list. (Without `LATERAL`, each
    sub-`SELECT` is evaluated independently and so cannot
    cross-reference any other `FROM` item.)

    `LATERAL` can also precede a function-call `FROM` item, but in this
    case it is a noise word, because the function expression can refer
    to earlier `FROM` items in any case.

    A `LATERAL` item can appear at top level in the `FROM` list, or
    within a `JOIN` tree. In the latter case it can also refer to any
    items that are on the left-hand side of a `JOIN` that it is on the
    right-hand side of.

    When a `FROM` item contains `LATERAL` cross-references, evaluation
    proceeds as follows: for each row of the `FROM` item providing the
    cross-referenced column(s), or set of rows of multiple `FROM` items
    providing the columns, the `LATERAL` item is evaluated using that
    row or row set\'s values of the columns. The resulting row(s) are
    joined as usual with the rows they were computed from. This is
    repeated for each row or set of rows from the column source
    table(s).

    The column source table(s) must be `INNER` or `LEFT` joined to the
    `LATERAL` item, else there would not be a well-defined set of rows
    from which to compute each set of rows for the `LATERAL` item. Thus,
    although a construct such as `X RIGHT JOIN
            LATERAL Y` is syntactically valid, it is not actually
    allowed for Y to reference X.

`WHERE` Clause {#sql-where}
--------------

The optional `WHERE` clause has the general form WHERE condition where
condition is any expression that evaluates to a result of type
`boolean`. Any row that does not satisfy this condition will be
eliminated from the output. A row satisfies the condition if it returns
true when the actual row values are substituted for any variable
references.

`GROUP BY` Clause {#sql-groupby}
-----------------

The optional `GROUP BY` clause has the general form GROUP BY \[ ALL \|
DISTINCT \] grouping\_element \[, \...\]

`GROUP BY` will condense into a single row all selected rows that share
the same values for the grouped expressions. An expression used inside a
grouping\_element can be an input column name, or the name or ordinal
number of an output column (`SELECT` list item), or an arbitrary
expression formed from input-column values. In case of ambiguity, a
`GROUP BY` name will be interpreted as an input-column name rather than
an output column name.

If any of `GROUPING SETS`, `ROLLUP` or `CUBE` are present as grouping
elements, then the `GROUP BY` clause as a whole defines some number of
independent grouping sets. The effect of this is equivalent to
constructing a `UNION ALL` between subqueries with the individual
grouping sets as their `GROUP BY` clauses. The optional `DISTINCT`
clause removes duplicate sets before processing; it does *not* transform
the `UNION ALL` into a `UNION DISTINCT`. For further details on the
handling of grouping sets see [???](#queries-grouping-sets).

Aggregate functions, if any are used, are computed across all rows
making up each group, producing a separate value for each group. (If
there are aggregate functions but no `GROUP BY` clause, the query is
treated as having a single group comprising all the selected rows.) The
set of rows fed to each aggregate function can be further filtered by
attaching a `FILTER` clause to the aggregate function call; see
[???](#syntax-aggregates) for more information. When a `FILTER` clause
is present, only those rows matching it are included in the input to
that aggregate function.

When `GROUP BY` is present, or any aggregate functions are present, it
is not valid for the `SELECT` list expressions to refer to ungrouped
columns except within aggregate functions or when the ungrouped column
is functionally dependent on the grouped columns, since there would
otherwise be more than one possible value to return for an ungrouped
column. A functional dependency exists if the grouped columns (or a
subset thereof) are the primary key of the table containing the
ungrouped column.

Keep in mind that all aggregate functions are evaluated before
evaluating any "scalar" expressions in the `HAVING` clause or `SELECT`
list. This means that, for example, a `CASE` expression cannot be used
to skip evaluation of an aggregate function; see
[???](#syntax-express-eval).

Currently, `FOR NO KEY UPDATE`, `FOR UPDATE`, `FOR SHARE` and
`FOR KEY SHARE` cannot be specified with `GROUP BY`.

`HAVING` Clause {#sql-having}
---------------

The optional `HAVING` clause has the general form HAVING condition where
condition is the same as specified for the `WHERE` clause.

`HAVING` eliminates group rows that do not satisfy the condition.
`HAVING` is different from `WHERE`: `WHERE` filters individual rows
before the application of `GROUP
    BY`, while `HAVING` filters group rows created by `GROUP BY`. Each
column referenced in condition must unambiguously reference a grouping
column, unless the reference appears within an aggregate function or the
ungrouped column is functionally dependent on the grouping columns.

The presence of `HAVING` turns a query into a grouped query even if
there is no `GROUP BY` clause. This is the same as what happens when the
query contains aggregate functions but no `GROUP BY` clause. All the
selected rows are considered to form a single group, and the `SELECT`
list and `HAVING` clause can only reference table columns from within
aggregate functions. Such a query will emit a single row if the `HAVING`
condition is true, zero rows if it is not true.

Currently, `FOR NO KEY UPDATE`, `FOR UPDATE`, `FOR SHARE` and
`FOR KEY SHARE` cannot be specified with `HAVING`.

`WINDOW` Clause {#sql-window}
---------------

The optional `WINDOW` clause has the general form WINDOW window\_name AS
( window\_definition ) \[, \...\] where window\_name is a name that can
be referenced from `OVER` clauses or subsequent window definitions, and
window\_definition is \[ existing\_window\_name \] \[ PARTITION BY
expression \[, \...\] \] \[ ORDER BY expression \[ ASC \| DESC \| USING
operator \] \[ NULLS { FIRST \| LAST } \] \[, \...\] \] \[ frame\_clause
\]

If an existing\_window\_name is specified it must refer to an earlier
entry in the `WINDOW` list; the new window copies its partitioning
clause from that entry, as well as its ordering clause if any. In this
case the new window cannot specify its own `PARTITION BY` clause, and it
can specify `ORDER BY` only if the copied window does not have one. The
new window always uses its own frame clause; the copied window must not
specify a frame clause.

The elements of the `PARTITION BY` list are interpreted in much the same
fashion as elements of a [`GROUP BY`](#sql-groupby) clause, except that
they are always simple expressions and never the name or number of an
output column. Another difference is that these expressions can contain
aggregate function calls, which are not allowed in a regular `GROUP BY`
clause. They are allowed here because windowing occurs after grouping
and aggregation.

Similarly, the elements of the `ORDER BY` list are interpreted in much
the same fashion as elements of a statement-level
[`ORDER BY`](#sql-orderby) clause, except that the expressions are
always taken as simple expressions and never the name or number of an
output column.

The optional frame\_clause defines the window frame for window functions
that depend on the frame (not all do). The window frame is a set of
related rows for each row of the query (called the current row). The
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 If frame\_end
is omitted it defaults to `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.

The default framing option is `RANGE UNBOUNDED PRECEDING`, which is the
same as `RANGE BETWEEN UNBOUNDED PRECEDING AND
    CURRENT ROW`; it sets the frame to be all rows from the partition
start up through the current row\'s last peer (a row that the window\'s
`ORDER BY` clause considers equivalent to the current row; all rows are
peers if there is no `ORDER BY`). In general, `UNBOUNDED PRECEDING`
means that the frame starts with the first row of the partition, and
similarly `UNBOUNDED FOLLOWING` means that the frame ends with the last
row of the partition, regardless of `RANGE`, `ROWS` or `GROUPS` mode. In
`ROWS` mode, `CURRENT ROW` means that the frame starts or ends with the
current row; but in `RANGE` or `GROUPS` mode it means that the frame
starts or ends with the current row\'s first or last peer in the
`ORDER BY` ordering. The offset `PRECEDING` and offset `FOLLOWING`
options vary in meaning depending on the frame mode. In `ROWS` mode, the
offset is an integer indicating that the frame starts or ends that many
rows before or after the current row. In `GROUPS` mode, the offset is an
integer indicating that the frame starts or ends that many peer groups
before or after the current row\'s peer group, where a peer group is a
group of rows that are equivalent according to the window\'s `ORDER BY`
clause. In `RANGE` mode, use of an offset option requires that there be
exactly one `ORDER BY` column in the window definition. Then the frame
contains those rows whose ordering column value is no more than offset
less than (for `PRECEDING`) or more than (for `FOLLOWING`) the current
row\'s ordering column value. In these cases the data type of the offset
expression depends 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`. In all
these cases, the value of the offset must be non-null and non-negative.
Also, while the offset does not have to be a simple constant, it cannot
contain variables, aggregate functions, or window functions.

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.

Beware that the `ROWS` mode can produce unpredictable results if the
`ORDER BY` ordering does not order the rows uniquely. The `RANGE` and
`GROUPS` modes are designed to ensure that rows that are peers in the
`ORDER BY` ordering are treated alike: all rows of a given peer group
will be in the frame or excluded from it.

The purpose of a `WINDOW` clause is to specify the behavior of window
functions appearing in the query\'s [`SELECT` list](#sql-select-list) or
[`ORDER BY`](#sql-orderby) clause. These functions can reference the
`WINDOW` clause entries by name in their `OVER` clauses. A `WINDOW`
clause entry does not have to be referenced anywhere, however; if it is
not used in the query it is simply ignored. It is possible to use window
functions without any `WINDOW` clause at all, since a window function
call can specify its window definition directly in its `OVER` clause.
However, the `WINDOW` clause saves typing when the same window
definition is needed for more than one window function.

Currently, `FOR NO KEY UPDATE`, `FOR UPDATE`, `FOR SHARE` and
`FOR KEY SHARE` cannot be specified with `WINDOW`.

Window functions are described in detail in [???](#tutorial-window),
[???](#syntax-window-functions), and [???](#queries-window).

`SELECT` List {#sql-select-list}
-------------

The `SELECT` list (between the key words `SELECT` and `FROM`) specifies
expressions that form the output rows of the `SELECT` statement. The
expressions can (and usually do) refer to columns computed in the `FROM`
clause.

Just as in a table, every output column of a `SELECT` has a name. In a
simple `SELECT` this name is just used to label the column for display,
but when the `SELECT` is a sub-query of a larger query, the name is seen
by the larger query as the column name of the virtual table produced by
the sub-query. To specify the name to use for an output column, write
`AS` output\_name after the column\'s expression. (You can omit `AS`,
but only if the desired output name does not match any PostgreSQL
keyword (see [???](#sql-keywords-appendix)). For protection against
possible future keyword additions, it is recommended that you always
either write `AS` or double-quote the output name.) If you do not
specify a column name, a name is chosen automatically by PostgreSQL. If
the column\'s expression is a simple column reference then the chosen
name is the same as that column\'s name. In more complex cases a
function or type name may be used, or the system may fall back on a
generated name such as `?column?`.

An output column\'s name can be used to refer to the column\'s value in
`ORDER BY` and `GROUP BY` clauses, but not in the `WHERE` or `HAVING`
clauses; there you must write out the expression instead.

Instead of an expression, `*` can be written in the output list as a
shorthand for all the columns of the selected rows. Also, you can write
`table_name.*` as a shorthand for the columns coming from just that
table. In these cases it is not possible to specify new names with `AS`;
the output column names will be the same as the table columns\' names.

According to the SQL standard, the expressions in the output list should
be computed before applying `DISTINCT`, `ORDER
    BY`, or `LIMIT`. This is obviously necessary when using `DISTINCT`,
since otherwise it\'s not clear what values are being made distinct.
However, in many cases it is convenient if output expressions are
computed after `ORDER
    BY` and `LIMIT`; particularly if the output list contains any
volatile or expensive functions. With that behavior, the order of
function evaluations is more intuitive and there will not be evaluations
corresponding to rows that never appear in the output. PostgreSQL will
effectively evaluate output expressions after sorting and limiting, so
long as those expressions are not referenced in `DISTINCT`, `ORDER BY`
or `GROUP BY`. (As a counterexample, `SELECT
    f(x) FROM tab ORDER BY 1` clearly must evaluate `f(x)` before
sorting.) Output expressions that contain set-returning functions are
effectively evaluated after sorting and before limiting, so that `LIMIT`
will act to cut off the output from a set-returning function.

> **Note**
>
> PostgreSQL versions before 9.6 did not provide any guarantees about
> the timing of evaluation of output expressions versus sorting and
> limiting; it depended on the form of the chosen query plan.

`DISTINCT` Clause {#sql-distinct}
-----------------

If `SELECT DISTINCT` is specified, all duplicate rows are removed from
the result set (one row is kept from each group of duplicates).
`SELECT ALL` specifies the opposite: all rows are kept; that is the
default.

`SELECT DISTINCT ON ( expression [, ...] )` keeps only the first row of
each set of rows where the given expressions evaluate to equal. The
`DISTINCT ON` expressions are interpreted using the same rules as for
`ORDER BY` (see above). Note that the "first row" of each set is
unpredictable unless `ORDER
    BY` is used to ensure that the desired row appears first. For
example:

    SELECT DISTINCT ON (location) location, time, report
        FROM weather_reports
        ORDER BY location, time DESC;

retrieves the most recent weather report for each location. But if we
had not used `ORDER BY` to force descending order of time values for
each location, we\'d have gotten a report from an unpredictable time for
each location.

The `DISTINCT ON` expression(s) must match the leftmost `ORDER BY`
expression(s). The `ORDER BY` clause will normally contain additional
expression(s) that determine the desired precedence of rows within each
`DISTINCT ON` group.

Currently, `FOR NO KEY UPDATE`, `FOR UPDATE`, `FOR SHARE` and
`FOR KEY SHARE` cannot be specified with `DISTINCT`.

`UNION` Clause {#sql-union}
--------------

The `UNION` clause has this general form: select\_statement UNION \[ ALL
\| DISTINCT \] select\_statementselect\_statement is any `SELECT`
statement without an `ORDER
    BY`, `LIMIT`, `FOR NO KEY UPDATE`, `FOR UPDATE`, `FOR SHARE`, or
`FOR KEY SHARE` clause. (`ORDER BY` and `LIMIT` can be attached to a
subexpression if it is enclosed in parentheses. Without parentheses,
these clauses will be taken to apply to the result of the `UNION`, not
to its right-hand input expression.)

The `UNION` operator computes the set union of the rows returned by the
involved `SELECT` statements. A row is in the set union of two result
sets if it appears in at least one of the result sets. The two `SELECT`
statements that represent the direct operands of the `UNION` must
produce the same number of columns, and corresponding columns must be of
compatible data types.

The result of `UNION` does not contain any duplicate rows unless the
`ALL` option is specified. `ALL` prevents elimination of duplicates.
(Therefore, `UNION ALL` is usually significantly quicker than `UNION`;
use `ALL` when you can.) `DISTINCT` can be written to explicitly specify
the default behavior of eliminating duplicate rows.

Multiple `UNION` operators in the same `SELECT` statement are evaluated
left to right, unless otherwise indicated by parentheses.

Currently, `FOR NO KEY UPDATE`, `FOR UPDATE`, `FOR SHARE` and
`FOR KEY SHARE` cannot be specified either for a `UNION` result or for
any input of a `UNION`.

`INTERSECT` Clause {#sql-intersect}
------------------

The `INTERSECT` clause has this general form: select\_statement
INTERSECT \[ ALL \| DISTINCT \] select\_statementselect\_statement is
any `SELECT` statement without an `ORDER
    BY`, `LIMIT`, `FOR NO KEY UPDATE`, `FOR UPDATE`, `FOR SHARE`, or
`FOR KEY SHARE` clause.

The `INTERSECT` operator computes the set intersection of the rows
returned by the involved `SELECT` statements. A row is in the
intersection of two result sets if it appears in both result sets.

The result of `INTERSECT` does not contain any duplicate rows unless the
`ALL` option is specified. With `ALL`, a row that has m duplicates in
the left table and n duplicates in the right table will appear min(m,n)
times in the result set. `DISTINCT` can be written to explicitly specify
the default behavior of eliminating duplicate rows.

Multiple `INTERSECT` operators in the same `SELECT` statement are
evaluated left to right, unless parentheses dictate otherwise.
`INTERSECT` binds more tightly than `UNION`. That is,
`A UNION B INTERSECT
    C` will be read as `A UNION (B INTERSECT
    C)`.

Currently, `FOR NO KEY UPDATE`, `FOR UPDATE`, `FOR SHARE` and
`FOR KEY SHARE` cannot be specified either for an `INTERSECT` result or
for any input of an `INTERSECT`.

`EXCEPT` Clause {#sql-except}
---------------

The `EXCEPT` clause has this general form: select\_statement EXCEPT \[
ALL \| DISTINCT \] select\_statementselect\_statement is any `SELECT`
statement without an `ORDER
    BY`, `LIMIT`, `FOR NO KEY UPDATE`, `FOR UPDATE`, `FOR SHARE`, or
`FOR KEY SHARE` clause.

The `EXCEPT` operator computes the set of rows that are in the result of
the left `SELECT` statement but not in the result of the right one.

The result of `EXCEPT` does not contain any duplicate rows unless the
`ALL` option is specified. With `ALL`, a row that has m duplicates in
the left table and n duplicates in the right table will appear
max(m-n,0) times in the result set. `DISTINCT` can be written to
explicitly specify the default behavior of eliminating duplicate rows.

Multiple `EXCEPT` operators in the same `SELECT` statement are evaluated
left to right, unless parentheses dictate otherwise. `EXCEPT` binds at
the same level as `UNION`.

Currently, `FOR NO KEY UPDATE`, `FOR UPDATE`, `FOR SHARE` and
`FOR KEY SHARE` cannot be specified either for an `EXCEPT` result or for
any input of an `EXCEPT`.

`ORDER BY` Clause {#sql-orderby}
-----------------

The optional `ORDER BY` clause has this general form: ORDER BY
expression \[ ASC \| DESC \| USING operator \] \[ NULLS { FIRST \| LAST
} \] \[, \...\] The `ORDER BY` clause causes the result rows to be
sorted according to the specified expression(s). If two rows are equal
according to the leftmost expression, they are compared according to the
next expression and so on. If they are equal according to all specified
expressions, they are returned in an implementation-dependent order.

Each expression can be the name or ordinal number of an output column
(`SELECT` list item), or it can be an arbitrary expression formed from
input-column values.

The ordinal number refers to the ordinal (left-to-right) position of the
output column. This feature makes it possible to define an ordering on
the basis of a column that does not have a unique name. This is never
absolutely necessary because it is always possible to assign a name to
an output column using the `AS` clause.

It is also possible to use arbitrary expressions in the `ORDER BY`
clause, including columns that do not appear in the `SELECT` output
list. Thus the following statement is valid:

    SELECT name FROM distributors ORDER BY code;

A limitation of this feature is that an `ORDER BY` clause applying to
the result of a `UNION`, `INTERSECT`, or `EXCEPT` clause can only
specify an output column name or number, not an expression.

If an `ORDER BY` expression is a simple name that matches both an output
column name and an input column name, `ORDER BY` will interpret it as
the output column name. This is the opposite of the choice that
`GROUP BY` will make in the same situation. This inconsistency is made
to be compatible with the SQL standard.

Optionally one can add the key word `ASC` (ascending) or `DESC`
(descending) after any expression in the `ORDER BY` clause. If not
specified, `ASC` is assumed by default. Alternatively, a specific
ordering operator name can be specified in the `USING` clause. An
ordering operator must be a less-than or greater-than member of some
B-tree operator family. `ASC` is usually equivalent to `USING <` and
`DESC` is usually equivalent to `USING >`. (But the creator of a
user-defined data type can define exactly what the default sort ordering
is, and it might correspond to operators with other names.)

If `NULLS LAST` is specified, null values sort after all non-null
values; if `NULLS FIRST` is specified, null values sort before all
non-null values. If neither is specified, the default behavior is
`NULLS LAST` when `ASC` is specified or implied, and `NULLS FIRST` when
`DESC` is specified (thus, the default is to act as though nulls are
larger than non-nulls). When `USING` is specified, the default nulls
ordering depends on whether the operator is a less-than or greater-than
operator.

Note that ordering options apply only to the expression they follow; for
example `ORDER BY x, y DESC` does not mean the same thing as
`ORDER BY x DESC, y DESC`.

Character-string data is sorted according to the collation that applies
to the column being sorted. That can be overridden at need by including
a `COLLATE` clause in the expression, for example
`ORDER BY mycolumn COLLATE "en_US"`. For more information see
[???](#sql-syntax-collate-exprs) and [???](#collation).

`LIMIT` Clause {#sql-limit}
--------------

The `LIMIT` clause consists of two independent sub-clauses: LIMIT {
count \| ALL } OFFSET start The parameter count specifies the maximum
number of rows to return, while start specifies the number of rows to
skip before starting to return rows. When both are specified, start rows
are skipped before starting to count the count rows to be returned.

If the count expression evaluates to NULL, it is treated as `LIMIT ALL`,
i.e., no limit. If start evaluates to NULL, it is treated the same as
`OFFSET 0`.

SQL:2008 introduced a different syntax to achieve the same result, which
PostgreSQL also supports. It is: OFFSET start { ROW \| ROWS } FETCH {
FIRST \| NEXT } \[ count \] { ROW \| ROWS } { ONLY \| WITH TIES } In
this syntax, the start or count value is required by the standard to be
a literal constant, a parameter, or a variable name; as a PostgreSQL
extension, other expressions are allowed, but will generally need to be
enclosed in parentheses to avoid ambiguity. If count is omitted in a
`FETCH` clause, it defaults to 1. The `WITH TIES` option is used to
return any additional rows that tie for the last place in the result set
according to the `ORDER BY` clause; `ORDER BY` is mandatory in this
case, and `SKIP LOCKED` is not allowed. `ROW` and `ROWS` as well as
`FIRST` and `NEXT` are noise words that don\'t influence the effects of
these clauses. According to the standard, the `OFFSET` clause must come
before the `FETCH` clause if both are present; but PostgreSQL is laxer
and allows either order.

When using `LIMIT`, it is a good idea to use an `ORDER BY` clause that
constrains the result rows into a unique order. Otherwise you will get
an unpredictable subset of the query\'s rows MDASH you might be asking
for the tenth through twentieth rows, but tenth through twentieth in
what ordering? You don\'t know what ordering unless you specify
`ORDER BY`.

The query planner takes `LIMIT` into account when generating a query
plan, so you are very likely to get different plans (yielding different
row orders) depending on what you use for `LIMIT` and `OFFSET`. Thus,
using different `LIMIT`/`OFFSET` values to select different subsets of a
query result *will give inconsistent results* unless you enforce a
predictable result ordering with `ORDER BY`. This is not a bug; it is an
inherent consequence of the fact that SQL does not promise to deliver
the results of a query in any particular order unless `ORDER BY` is used
to constrain the order.

It is even possible for repeated executions of the same `LIMIT` query to
return different subsets of the rows of a table, if there is not an
`ORDER BY` to enforce selection of a deterministic subset. Again, this
is not a bug; determinism of the results is simply not guaranteed in
such a case.

The Locking Clause {#sql-for-update-share}
------------------

`FOR UPDATE`, `FOR NO KEY UPDATE`, `FOR SHARE` and `FOR KEY SHARE` are
locking clauses; they affect how `SELECT` locks rows as they are
obtained from the table.

The locking clause has the general form FOR lock\_strength \[ OF
table\_name \[, \...\] \] \[ NOWAIT \| SKIP LOCKED \] where
lock\_strength can be one of UPDATE NO KEY UPDATE SHARE KEY SHARE

For more information on each row-level lock mode, refer to
[???](#locking-rows).

To prevent the operation from waiting for other transactions to commit,
use either the `NOWAIT` or `SKIP LOCKED` option. With `NOWAIT`, the
statement reports an error, rather than waiting, if a selected row
cannot be locked immediately. With `SKIP LOCKED`, any selected rows that
cannot be immediately locked are skipped. Skipping locked rows provides
an inconsistent view of the data, so this is not suitable for general
purpose work, but can be used to avoid lock contention with multiple
consumers accessing a queue-like table. Note that `NOWAIT` and
`SKIP LOCKED` apply only to the row-level lock(s) MDASH the required
`ROW SHARE` table-level lock is still taken in the ordinary way (see
[???](#mvcc)). You can use [`LOCK`](#sql-lock) with the `NOWAIT` option
first, if you need to acquire the table-level lock without waiting.

If specific tables are named in a locking clause, then only rows coming
from those tables are locked; any other tables used in the `SELECT` are
simply read as usual. A locking clause without a table list affects all
tables used in the statement. If a locking clause is applied to a view
or sub-query, it affects all tables used in the view or sub-query.
However, these clauses do not apply to `WITH` queries referenced by the
primary query. If you want row locking to occur within a `WITH` query,
specify a locking clause within the `WITH` query.

Multiple locking clauses can be written if it is necessary to specify
different locking behavior for different tables. If the same table is
mentioned (or implicitly affected) by more than one locking clause, then
it is processed as if it was only specified by the strongest one.
Similarly, a table is processed as `NOWAIT` if that is specified in any
of the clauses affecting it. Otherwise, it is processed as `SKIP LOCKED`
if that is specified in any of the clauses affecting it.

The locking clauses cannot be used in contexts where returned rows
cannot be clearly identified with individual table rows; for example
they cannot be used with aggregation.

When a locking clause appears at the top level of a `SELECT` query, the
rows that are locked are exactly those that are returned by the query;
in the case of a join query, the rows locked are those that contribute
to returned join rows. In addition, rows that satisfied the query
conditions as of the query snapshot will be locked, although they will
not be returned if they were updated after the snapshot and no longer
satisfy the query conditions. If a `LIMIT` is used, locking stops once
enough rows have been returned to satisfy the limit (but note that rows
skipped over by `OFFSET` will get locked). Similarly, if a locking
clause is used in a cursor\'s query, only rows actually fetched or
stepped past by the cursor will be locked.

When a locking clause appears in a sub-`SELECT`, the rows locked are
those returned to the outer query by the sub-query. This might involve
fewer rows than inspection of the sub-query alone would suggest, since
conditions from the outer query might be used to optimize execution of
the sub-query. For example,

    SELECT * FROM (SELECT * FROM mytable FOR UPDATE) ss WHERE col1 = 5;

will lock only rows having `col1 = 5`, even though that condition is not
textually within the sub-query.

Previous releases failed to preserve a lock which is upgraded by a later
savepoint. For example, this code:

    BEGIN;
    SELECT * FROM mytable WHERE key = 1 FOR UPDATE;
    SAVEPOINT s;
    UPDATE mytable SET ... WHERE key = 1;
    ROLLBACK TO s;

would fail to preserve the `FOR UPDATE` lock after the `ROLLBACK TO`.
This has been fixed in release 9.3.

> **Caution**
>
> It is possible for a `SELECT` command running at the `READ
>     COMMITTED` transaction isolation level and using `ORDER
>     BY` and a locking clause to return rows out of order. This is
> because `ORDER BY` is applied first. The command sorts the result, but
> might then block trying to obtain a lock on one or more of the rows.
> Once the `SELECT` unblocks, some of the ordering column values might
> have been modified, leading to those rows appearing to be out of order
> (though they are in order in terms of the original column values).
> This can be worked around at need by placing the `FOR UPDATE/SHARE`
> clause in a sub-query, for example
>
>     SELECT * FROM (SELECT * FROM mytable FOR UPDATE) ss ORDER BY column1;
>
> Note that this will result in locking all rows of mytable, whereas
> `FOR UPDATE` at the top level would lock only the actually returned
> rows. This can make for a significant performance difference,
> particularly if the `ORDER BY` is combined with `LIMIT` or other
> restrictions. So this technique is recommended only if concurrent
> updates of the ordering columns are expected and a strictly sorted
> result is required.
>
> At the `REPEATABLE READ` or `SERIALIZABLE` transaction isolation level
> this would cause a serialization failure (with a `SQLSTATE` of
> `'40001'`), so there is no possibility of receiving rows out of order
> under these isolation levels.

`TABLE` Command {#sql-table}
---------------

The command

    TABLE name

is equivalent to

    SELECT * FROM name

It can be used as a top-level command or as a space-saving syntax
variant in parts of complex queries. Only the `WITH`, `UNION`,
`INTERSECT`, `EXCEPT`, `ORDER BY`, `LIMIT`, `OFFSET`, `FETCH` and `FOR`
locking clauses can be used with `TABLE`; the `WHERE` clause and any
form of aggregation cannot be used.

Examples
========

To join the table `films` with the table `distributors`:

    SELECT f.title, f.did, d.name, f.date_prod, f.kind
        FROM distributors d, films f
        WHERE f.did = d.did

           title       | did |     name     | date_prod  |   kind
    -------------------+-----+--------------+------------+----------
     The Third Man     | 101 | British Lion | 1949-12-23 | Drama
     The African Queen | 101 | British Lion | 1951-08-11 | Romantic
     ...

To sum the column `len` of all films and group the results by `kind`:

    SELECT kind, sum(len) AS total FROM films GROUP BY kind;

       kind   | total
    ----------+-------
     Action   | 07:34
     Comedy   | 02:58
     Drama    | 14:28
     Musical  | 06:42
     Romantic | 04:38

To sum the column `len` of all films, group the results by `kind` and
show those group totals that are less than 5 hours:

    SELECT kind, sum(len) AS total
        FROM films
        GROUP BY kind
        HAVING sum(len) < interval '5 hours';

       kind   | total
    ----------+-------
     Comedy   | 02:58
     Romantic | 04:38

The following two examples are identical ways of sorting the individual
results according to the contents of the second column (`name`):

    SELECT * FROM distributors ORDER BY name;
    SELECT * FROM distributors ORDER BY 2;

     did |       name
    -----+------------------
     109 | 20th Century Fox
     110 | Bavaria Atelier
     101 | British Lion
     107 | Columbia
     102 | Jean Luc Godard
     113 | Luso films
     104 | Mosfilm
     103 | Paramount
     106 | Toho
     105 | United Artists
     111 | Walt Disney
     112 | Warner Bros.
     108 | Westward

The next example shows how to obtain the union of the tables
`distributors` and `actors`, restricting the results to those that begin
with the letter W in each table. Only distinct rows are wanted, so the
key word `ALL` is omitted.

    distributors:               actors:
     did |     name              id |     name
    -----+--------------        ----+----------------
     108 | Westward               1 | Woody Allen
     111 | Walt Disney            2 | Warren Beatty
     112 | Warner Bros.           3 | Walter Matthau
     ...                         ...

    SELECT distributors.name
        FROM distributors
        WHERE distributors.name LIKE 'W%'
    UNION
    SELECT actors.name
        FROM actors
        WHERE actors.name LIKE 'W%';

          name
    ----------------
     Walt Disney
     Walter Matthau
     Warner Bros.
     Warren Beatty
     Westward
     Woody Allen

This example shows how to use a function in the `FROM` clause, both with
and without a column definition list:

    CREATE FUNCTION distributors(int) RETURNS SETOF distributors AS $$
        SELECT * FROM distributors WHERE did = $1;
    $$ LANGUAGE SQL;

    SELECT * FROM distributors(111);
     did |    name
    -----+-------------
     111 | Walt Disney

    CREATE FUNCTION distributors_2(int) RETURNS SETOF record AS $$
        SELECT * FROM distributors WHERE did = $1;
    $$ LANGUAGE SQL;

    SELECT * FROM distributors_2(111) AS (f1 int, f2 text);
     f1  |     f2
    -----+-------------
     111 | Walt Disney

Here is an example of a function with an ordinality column added:

    SELECT * FROM unnest(ARRAY['a','b','c','d','e','f']) WITH ORDINALITY;
     unnest | ordinality
    --------+----------
     a      |        1
     b      |        2
     c      |        3
     d      |        4
     e      |        5
     f      |        6
    (6 rows)

This example shows how to use a simple `WITH` clause:

    WITH t AS (
        SELECT random() as x FROM generate_series(1, 3)
      )
    SELECT * FROM t
    UNION ALL
    SELECT * FROM t

             x          
    --------------------
      0.534150459803641
      0.520092216785997
     0.0735620250925422
      0.534150459803641
      0.520092216785997
     0.0735620250925422

Notice that the `WITH` query was evaluated only once, so that we got two
sets of the same three random values.

This example uses `WITH RECURSIVE` to find all subordinates (direct or
indirect) of the employee Mary, and their level of indirectness, from a
table that shows only direct subordinates:

    WITH RECURSIVE employee_recursive(distance, employee_name, manager_name) AS (
        SELECT 1, employee_name, manager_name
        FROM employee
        WHERE manager_name = 'Mary'
      UNION ALL
        SELECT er.distance + 1, e.employee_name, e.manager_name
        FROM employee_recursive er, employee e
        WHERE er.employee_name = e.manager_name
      )
    SELECT distance, employee_name FROM employee_recursive;

Notice the typical form of recursive queries: an initial condition,
followed by `UNION`, followed by the recursive part of the query. Be
sure that the recursive part of the query will eventually return no
tuples, or else the query will loop indefinitely. (See
[???](#queries-with) for more examples.)

This example uses `LATERAL` to apply a set-returning function
`get_product_names()` for each row of the manufacturers table:

    SELECT m.name AS mname, pname
    FROM manufacturers m, LATERAL get_product_names(m.id) pname;

Manufacturers not currently having any products would not appear in the
result, since it is an inner join. If we wished to include the names of
such manufacturers in the result, we could do:

    SELECT m.name AS mname, pname
    FROM manufacturers m LEFT JOIN LATERAL get_product_names(m.id) pname ON true;

Compatibility
=============

Of course, the `SELECT` statement is compatible with the SQL standard.
But there are some extensions and some missing features.

Omitted `FROM` Clauses
----------------------

PostgreSQL allows one to omit the `FROM` clause. It has a
straightforward use to compute the results of simple expressions:

    SELECT 2+2;

     ?column?
    ----------
            4

Some other SQL databases cannot do this except by introducing a dummy
one-row table from which to do the `SELECT`.

Empty `SELECT` Lists
--------------------

The list of output expressions after `SELECT` can be empty, producing a
zero-column result table. This is not valid syntax according to the SQL
standard. PostgreSQL allows it to be consistent with allowing
zero-column tables. However, an empty list is not allowed when
`DISTINCT` is used.

Omitting the `AS` Key Word
--------------------------

In the SQL standard, the optional key word `AS` can be omitted before an
output column name whenever the new column name is a valid column name
(that is, not the same as any reserved keyword). PostgreSQL is slightly
more restrictive: `AS` is required if the new column name matches any
keyword at all, reserved or not. Recommended practice is to use `AS` or
double-quote output column names, to prevent any possible conflict
against future keyword additions.

In `FROM` items, both the standard and PostgreSQL allow `AS` to be
omitted before an alias that is an unreserved keyword. But this is
impractical for output column names, because of syntactic ambiguities.

`ONLY` and Inheritance
----------------------

The SQL standard requires parentheses around the table name when writing
`ONLY`, for example `SELECT * FROM ONLY
    (tab1), ONLY (tab2) WHERE ...`. PostgreSQL considers these
parentheses to be optional.

PostgreSQL allows a trailing `*` to be written to explicitly specify the
non-`ONLY` behavior of including child tables. The standard does not
allow this.

(These points apply equally to all SQL commands supporting the `ONLY`
option.)

`TABLESAMPLE` Clause Restrictions
---------------------------------

The `TABLESAMPLE` clause is currently accepted only on regular tables
and materialized views. According to the SQL standard it should be
possible to apply it to any `FROM` item.

Function Calls in `FROM`
------------------------

PostgreSQL allows a function call to be written directly as a member of
the `FROM` list. In the SQL standard it would be necessary to wrap such
a function call in a sub-`SELECT`; that is, the syntax
`FROM func(...) alias` is approximately equivalent to
`FROM LATERAL (SELECT func(...)) alias`. Note that `LATERAL` is
considered to be implicit; this is because the standard requires
`LATERAL` semantics for an `UNNEST()` item in `FROM`. PostgreSQL treats
`UNNEST()` the same as other set-returning functions.

Namespace Available to `GROUP BY` and `ORDER BY`
------------------------------------------------

In the SQL-92 standard, an `ORDER BY` clause can only use output column
names or numbers, while a `GROUP
    BY` clause can only use expressions based on input column names.
PostgreSQL extends each of these clauses to allow the other choice as
well (but it uses the standard\'s interpretation if there is ambiguity).
PostgreSQL also allows both clauses to specify arbitrary expressions.
Note that names appearing in an expression will always be taken as
input-column names, not as output-column names.

SQL:1999 and later use a slightly different definition which is not
entirely upward compatible with SQL-92. In most cases, however,
PostgreSQL will interpret an `ORDER BY` or `GROUP
    BY` expression the same way SQL:1999 does.

Functional Dependencies
-----------------------

PostgreSQL recognizes functional dependency (allowing columns to be
omitted from `GROUP BY`) only when a table\'s primary key is included in
the `GROUP BY` list. The SQL standard specifies additional conditions
that should be recognized.

`LIMIT` and `OFFSET`
--------------------

The clauses `LIMIT` and `OFFSET` are PostgreSQL-specific syntax, also
used by MySQL. The SQL:2008 standard has introduced the clauses
`OFFSET ... FETCH {FIRST|NEXT}
    ...` for the same functionality, as shown above in [LIMIT
Clause](#sql-limit). This syntax is also used by IBM DB2. (Applications
written for Oracle frequently use a workaround involving the
automatically generated `rownum` column, which is not available in
PostgreSQL, to implement the effects of these clauses.)

`FOR NO KEY UPDATE`, `FOR UPDATE`, `FOR SHARE`, `FOR KEY SHARE`
---------------------------------------------------------------

Although `FOR UPDATE` appears in the SQL standard, the standard allows
it only as an option of `DECLARE CURSOR`. PostgreSQL allows it in any
`SELECT` query as well as in sub-`SELECT`s, but this is an extension.
The `FOR NO KEY UPDATE`, `FOR SHARE` and `FOR KEY SHARE` variants, as
well as the `NOWAIT` and `SKIP LOCKED` options, do not appear in the
standard.

Data-Modifying Statements in `WITH`
-----------------------------------

PostgreSQL allows `INSERT`, `UPDATE`, and `DELETE` to be used as `WITH`
queries. This is not found in the SQL standard.

Nonstandard Clauses
-------------------

`DISTINCT ON ( ... )` is an extension of the SQL standard.

`ROWS FROM( ... )` is an extension of the SQL standard.

The `MATERIALIZED` and `NOT
    MATERIALIZED` options of `WITH` are extensions of the SQL standard.
