SELECT
SELECT
SELECT, TABLE, WITH - retrieve rows from a table or view
Synopsis
[ WITH [ RECURSIVE ]with_query
[, ...] ] SELECT [ ALL | DISTINCT [ ON (expression
[, ...] ) ] ] [ * |expression
[ [ AS ]output_name
] [, ...] ] [ FROMfrom_item
[, ...] ] [ WHEREcondition
] [ GROUP BYgrouping_element
[, ...] ] [ HAVINGcondition
[, ...] ] [ WINDOWwindow_name
AS (window_definition
) [, ...] ] [ { UNION | INTERSECT | EXCEPT } [ ALL | DISTINCT ]select
] [ ORDER BYexpression
[ ASC | DESC | USINGoperator
] [ NULLS { FIRST | LAST } ] [, ...] ] [ LIMIT {count
| ALL } ] [ OFFSETstart
[ ROW | ROWS ] ] [ FETCH { FIRST | NEXT } [count
] { ROW | ROWS } ONLY ] [ FOR { UPDATE | NO KEY UPDATE | SHARE | KEY SHARE } [ OFtable_name
[, ...] ] [ NOWAIT | SKIP LOCKED ] [...] ] wherefrom_item
can be one of: [ ONLY ]table_name
[ * ] [ [ AS ]alias
[ (column_alias
[, ...] ) ] ] [ TABLESAMPLEsampling_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
[ ONjoin_condition
| USING (join_column
[, ...] ) ] andgrouping_element
can be one of: ( )expression
(expression
[, ...] ) ROLLUP ( {expression
| (expression
[, ...] ) } [, ...] ) CUBE ( {expression
| (expression
[, ...] ) } [, ...] ) GROUPING SETS (grouping_element
[, ...] ) andwith_query
is:with_query_name
[ (column_name
[, ...] ) ] AS (select
|values
|insert
|update
|delete
) TABLE [ ONLY ]table_name
[ * ]
Description
SELECT
retrieves rows from zero or more tables.
The general processing of
SELECT
is as follows:
-
All queries in the
WITH
list are computed. These effectively serve as temporary tables that can be referenced in theFROM
list. AWITH
query that is referenced more than once inFROM
is computed only once. (SeeWITH
Clause below.) -
All elements in the
FROM
list are computed. (Each element in theFROM
list is a real or virtual table.) If more than one element is specified in theFROM
list, they are cross-joined together. (SeeFROM
Clause below.) -
If the
WHERE
clause is specified, all rows that do not satisfy the condition are eliminated from the output. (SeeWHERE
Clause below.) -
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 theHAVING
clause is present, it eliminates groups that do not satisfy the given condition. (SeeGROUP BY
Clause andHAVING
Clause below.) -
The actual output rows are computed using the
SELECT
output expressions for each selected row or row group. (SeeSELECT
List below.) -
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. (SeeDISTINCT
Clause below.) -
Using the operators
UNION
,INTERSECT
, andEXCEPT
, the output of more than oneSELECT
statement can be combined to form a single result set. TheUNION
operator returns all rows that are in one or both of the result sets. TheINTERSECT
operator returns all rows that are strictly in both result sets. TheEXCEPT
operator returns the rows that are in the first result set but not in the second. In all three cases, duplicate rows are eliminated unlessALL
is specified. The noise wordDISTINCT
can be added to explicitly specify eliminating duplicate rows. Notice thatDISTINCT
is the default behavior here, even thoughALL
is the default forSELECT
itself. (SeeUNION
Clause ,INTERSECT
Clause , andEXCEPT
Clause below.) -
If the
ORDER BY
clause is specified, the returned rows are sorted in the specified order. IfORDER BY
is not given, the rows are returned in whatever order the system finds fastest to produce. (SeeORDER BY
Clause below.) -
If the
LIMIT
(orFETCH FIRST
) orOFFSET
clause is specified, theSELECT
statement only returns a subset of the result rows. (SeeLIMIT
Clause below.) -
If
FOR UPDATE
,FOR NO KEY UPDATE
,FOR SHARE
orFOR KEY SHARE
is specified, theSELECT
statement locks the selected rows against concurrent updates. (See The Locking Clause 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
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
Section 7.8
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.
A key property of
WITH
queries is that they
are 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.
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 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.
See Section 7.8 for additional information.
FROM
Clause
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. IfONLY
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 givenFROM foo AS f
, the remainder of theSELECT
must refer to thisFROM
item asf
notfoo
. 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 atable_name
indicates that the specifiedsampling_method
should be used to retrieve a subset of the rows in that table. This sampling precedes the application of any other filters such asWHERE
clauses. The standard PostgreSQL distribution includes two sampling methods,BERNOULLI
andSYSTEM
, and other sampling methods can be installed in the database via extensions.The
BERNOULLI
andSYSTEM
sampling methods each accept a singleargument
which is the fraction of the table to sample, expressed as a percentage between 0 and 100. This argument can be anyreal
-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. TheBERNOULLI
method scans the whole table and selects or ignores individual rows independently with the specified probability. TheSYSTEM
method does block-level sampling with each block having the specified chance of being selected; all rows in each selected block are returned. TheSYSTEM
method is significantly faster than theBERNOULLI
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 aseed
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 andargument
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. IfREPEATABLE
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 acceptREPEATABLE
, and will always produce new samples on each use. -
select
-
A sub-
SELECT
can appear in theFROM
clause. This acts as though its output were created as a temporary table for the duration of this singleSELECT
command. Note that the sub-SELECT
must be surrounded by parentheses, and an alias must be provided for it. A 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, theWITH
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 singleSELECT
command. When the optionalWITH ORDINALITY
clause is added to the function call, a new column is appended after all the function's output columns with numbering for each row.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 column added by
ORDINALITY
if present.Multiple function calls can be combined into a single
FROM
-clause item by surrounding them withROWS 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 wordAS
must be present, followed by a column definition list in the form(
. The column definition list must match the actual number and types of columns returned by the function.column_name
data_type
[ , ... ])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 insideROWS FROM( ... )
. A column definition list can be placed after theROWS FROM( ... )
construct only if there's just a single function and noWITH ORDINALITY
clause.To use
ORDINALITY
together with a column definition list, you must use theROWS FROM( ... )
syntax and put the column definition list insideROWS FROM( ... )
. -
join_type
-
One of
-
[ INNER ] JOIN
-
LEFT [ OUTER ] JOIN
-
RIGHT [ OUTER ] JOIN
-
FULL [ OUTER ] JOIN
-
CROSS JOIN
For the
INNER
andOUTER
join types, a join condition must be specified, namely exactly one ofNATURAL
,ON
, orjoin_condition
USING (
. See below for the meaning. Forjoin_column
[, ...])CROSS JOIN
, none of these clauses can appear.A
JOIN
clause combines twoFROM
items, which for convenience we will refer to as " tables " , though in reality they can be any type ofFROM
item. Use parentheses if necessary to determine the order of nesting. In the absence of parentheses,JOIN
s nest left-to-right. In any caseJOIN
binds more tightly than the commas separatingFROM
-list items.CROSS JOIN
andINNER JOIN
produce a simple Cartesian product, the same result as you get from listing the two tables at the top level ofFROM
, but restricted by the join condition (if any).CROSS JOIN
is equivalent toINNER 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 plainFROM
andWHERE
.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 theJOIN
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 aLEFT 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 typeboolean
(similar to aWHERE
clause) that specifies which rows in a join are considered to match. -
USING (
join_column
[, ...] ) -
A clause of the form
USING ( a, b, ... )
is shorthand forON 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. -
NATURAL
-
NATURAL
is shorthand for aUSING
list that mentions all columns in the two tables that have matching names. If there are no common column names,NATURAL
is equivalent toON TRUE
. -
LATERAL
-
The
LATERAL
key word can precede a sub-SELECT
FROM
item. This allows the sub-SELECT
to refer to columns ofFROM
items that appear before it in theFROM
list. (WithoutLATERAL
, each sub-SELECT
is evaluated independently and so cannot cross-reference any otherFROM
item.)LATERAL
can also precede a function-callFROM
item, but in this case it is a noise word, because the function expression can refer to earlierFROM
items in any case.A
LATERAL
item can appear at top level in theFROM
list, or within aJOIN
tree. In the latter case it can also refer to any items that are on the left-hand side of aJOIN
that it is on the right-hand side of.When a
FROM
item containsLATERAL
cross-references, evaluation proceeds as follows: for each row of theFROM
item providing the cross-referenced column(s), or set of rows of multipleFROM
items providing the columns, theLATERAL
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
orLEFT
joined to theLATERAL
item, else there would not be a well-defined set of rows from which to compute each set of rows for theLATERAL
item. Thus, although a construct such asX
RIGHT JOIN LATERALY
Y
to referenceX
.
WHERE
Clause
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
The optional
GROUP BY
clause has the general form
GROUP BY 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. For further details on the handling
of grouping sets see
Section 7.2.4
.
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
Section 4.2.7
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
Section 4.2.14
.
Currently,
FOR NO KEY UPDATE
,
FOR UPDATE
,
FOR SHARE
and
FOR KEY SHARE
cannot be
specified with
GROUP BY
.
HAVING
Clause
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
The optional
WINDOW
clause has the general form
WINDOWwindow_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 BYexpression
[, ...] ] [ ORDER BYexpression
[ ASC | DESC | USINGoperator
] [ 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
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 an
ORDER BY
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 }frame_start
{ RANGE | ROWS } BETWEENframe_start
ANDframe_end
where
frame_start
and
frame_end
can be
one of
UNBOUNDED PRECEDINGvalue
PRECEDING CURRENT ROWvalue
FOLLOWING UNBOUNDED FOLLOWING
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 than the
frame_start
choice - for example
RANGE BETWEEN CURRENT ROW AND
is not allowed.
value
PRECEDING
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
ORDER
BY
considers equivalent to the current row, or all rows 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
or
ROWS
mode). In
ROWS
mode,
CURRENT ROW
means that the frame starts or ends with the current row; but in
RANGE
mode it means that the frame starts or ends with
the current row's first or last peer in the
ORDER BY
ordering.
The
value
PRECEDING
and
value
FOLLOWING
cases are currently only
allowed in
ROWS
mode. They indicate that the frame starts
or ends with the row that many rows before or after the current row.
value
must be an integer expression not
containing any variables, aggregate functions, or window functions.
The value must not be null or negative; but it can be zero, which
selects the current row itself.
Beware that the
ROWS
options can produce unpredictable
results if the
ORDER BY
ordering does not order the rows
uniquely. The
RANGE
options are designed to ensure that
rows that are peers in the
ORDER BY
ordering are treated
alike; all peer rows will be in the same frame.
The purpose of a
WINDOW
clause is to specify the
behavior of
window functions
appearing in the query's
SELECT
List
or
ORDER BY
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 Section 3.5 , Section 4.2.8 , and Section 7.2.5 .
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
Appendix C
). 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
as a
shorthand for the columns coming from just that table. In these
cases it is not possible to specify new names with
table_name
.*
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
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 (
keeps only the first row of each set of rows where the given
expressions evaluate to equal. The
expression
[, ...] )
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
The
UNION
clause has this general form:
select_statement
UNION [ ALL | DISTINCT ]select_statement
select_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
The
INTERSECT
clause has this general form:
select_statement
INTERSECT [ ALL | DISTINCT ]select_statement
select_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
The
EXCEPT
clause has this general form:
select_statement
EXCEPT [ ALL | DISTINCT ]select_statement
select_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
The optional
ORDER BY
clause has this general form:
ORDER BYexpression
[ ASC | DESC | USINGoperator
] [ 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
Section 4.2.10
and
Section 23.2
.
LIMIT
Clause
The
LIMIT
clause consists of two independent
sub-clauses:
LIMIT {count
| ALL } OFFSETstart
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:
OFFSETstart
{ ROW | ROWS } FETCH { FIRST | NEXT } [count
] { ROW | ROWS } ONLY
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.
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 - 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
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
FORlock_strength
[ OFtable_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 Section 13.3.2 .
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) - the required
ROW SHARE
table-level lock is still taken in the ordinary way (see
Chapter 13
). You can use
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
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
Section 7.8
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
.
Note that if a
FROM
clause is not specified,
the query cannot reference any database tables. For example, the
following query is invalid:
SELECT distributors.* WHERE distributors.name = 'Westward';
PostgreSQL
releases prior to
8.1 would accept queries of this form, and add an implicit entry
to the query's
FROM
clause for each table
referenced by the query. This is no longer allowed.
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
is approximately equivalent to
func
(...)
alias
FROM LATERAL (SELECT
.
Note that
func
(...))
alias
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.
WINDOW
Clause Restrictions
The SQL standard provides additional options for the window
frame_clause
.
PostgreSQL
currently supports only the
options listed above.
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
. 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.