select
SELECT() SQL Commands SELECT()
NAME
SELECT - retrieve rows from a table or view
SYNOPSIS
SELECT [ ALL | DISTINCT [ ON ( expression [, ...] ) ] ]
* | expression [ AS output_name ] [, ...]
[ FROM from_item [, ...] ]
[ WHERE condition ]
[ GROUP BY expression [, ...] ]
[ HAVING condition [, ...] ]
[ { UNION | INTERSECT | EXCEPT } [ ALL ] select ]
[ ORDER BY expression [ ASC | DESC | USING operator ] [, ...] ]
[ LIMIT { count | ALL } ]
[ OFFSET start ]
[ FOR UPDATE [ OF table_name [, ...] ] ]
where from_item can be one of:
[ ONLY ] table_name [ * ] [ [ AS ] alias [ ( column_alias [, ...] ) ] ]
( select ) [ AS ] alias [ ( column_alias [, ...] ) ]
function_name ( [ argument [, ...] ] ) [ AS ] alias [ ( column_alias [, ...] | column_definition [, ...] ) ]
function_name ( [ argument [, ...] ] ) AS ( column_definition [, ...] )
from_item [ NATURAL ] join_type from_item [ ON join_condition | USING ( join_column [, ...] ) ]
DESCRIPTION
SELECT retrieves rows from one or more tables. The general processing
of SELECT is as follows:
1. 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 [select(7)] below.)
2. If the WHERE clause is specified, all rows that do not satisfy
the condition are eliminated from the output. (See WHERE Clause
[select(7)] below.)
3. If the GROUP BY clause is specified, the output is divided into
groups of rows that match on one or more values. If the HAVING
clause is present, it eliminates groups that do not satisfy the
given condition. (See GROUP BY Clause [select(7)] and HAVING
Clause [select(7)] below.)
4. 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. (See UNION Clause [select(7)], INTER-
SECT Clause [select(l)], and EXCEPT Clause [select(7)] below.)
5. The actual output rows are computed using the SELECT output
expressions for each selected row. (See SELECT List [select(7)]
below.)
6. 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 [select(7)] below.)
7. DISTINCT eliminates duplicate rows from the result. DISTINCT ON
eliminates rows that match on all the specified expressions.
ALL (the default) will return all candidate rows, including
duplicates. (See DISTINCT Clause [select(7)] below.)
8. If the LIMIT or OFFSET clause is specified, the SELECT state-
ment only returns a subset of the result rows. (See LIMIT
Clause [select(7)] below.)
9. The FOR UPDATE clause causes the SELECT statement to lock the
selected rows against concurrent updates. (See FOR UPDATE
Clause [select(7)] below.)
You must have SELECT privilege on a table to read its values. The use
of FOR UPDATE requires UPDATE privilege as well.
PARAMETERS
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 to restrict the returned rows to a small subset of the
Cartesian product.
FROM-clause elements can contain:
table_name
The name (optionally schema-qualified) of an existing table or
view. If ONLY is specified, only that table is scanned. If ONLY
is not specified, the table and all its descendant tables (if
any) are scanned. * can be appended to the table name to indi-
cate that descendant tables are to be scanned, but in the cur-
rent version, this is the default behavior. (In releases before
7.1, ONLY was the default behavior.) The default behavior can
be modified by changing the sql_inheritance configuration
option.
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 remain-
der 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.
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.
function_name
Function calls can appear in the FROM clause. (This is espe-
cially useful for functions that return result sets, but any
function can be used.) This acts as though its output were cre-
ated as a temporary table for the duration of this single
SELECT command. An alias may also be used. If an alias is writ-
ten, a column alias list can also be written to provide substi-
tute names for one or more attributes of the function’s compos-
ite return type. 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.
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 speci-
fied, namely exactly one of NATURAL, ON join_condition, or USING
(join_column [, ...]). See below for the meaning. For CROSS JOIN,
none of these clauses may appear.
A JOIN clause combines two FROM items. Use parentheses if necessary to
determine the order of nesting. In the absence of parentheses, JOINs
nest left-to-right. In any case JOIN binds more tightly than the com-
mas separating FROM items.
CROSS JOIN and INNER JOIN produce a simple Cartesian product, the same
result as you get from listing the two items at the top level of FROM,
but restricted by the join condition (if any). CROSS JOIN is equiva-
lent to INNER JOIN ON (TRUE), that is, no rows are removed by qualifi-
cation. 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 inputs.
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 [, ...])
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 equiva-
lent columns will be included in the join output, not both.
NATURAL
NATURAL is shorthand for a USING list that mentions all columns
in the two tables that have the same names.
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 elimi-
nated from the output. A row satisfies the condition if it returns
true when the actual row values are substituted for any variable ref-
erences.
GROUP BY CLAUSE
The optional GROUP BY clause has the general form
GROUP BY expression [, ...]
GROUP BY will condense into a single row all selected rows that share
the same values for the grouped expressions. expression 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-col-
umn values. In case of ambiguity, a GROUP BY name will be interpreted
as an input-column name rather than an output column name.
Aggregate functions, if any are used, are computed across all rows
making up each group, producing a separate value for each group
(whereas without GROUP BY, an aggregate produces a single value com-
puted across all the selected rows). When GROUP BY is present, it is
not valid for the SELECT list expressions to refer to ungrouped
columns except within aggregate functions, since there would be more
than one possible value to return for an ungrouped column.
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 ref-
erence a grouping column, unless the reference appears within an
aggregate function.
UNION CLAUSE
The UNION clause has this general form:
select_statement UNION [ ALL ] select_statement
select_statement is any SELECT statement without an ORDER BY, LIMIT,
or FOR UPDATE clause. (ORDER BY and LIMIT can be attached to a subex-
pression 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 pro-
duce 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.
Multiple UNION operators in the same SELECT statement are evaluated
left to right, unless otherwise indicated by parentheses.
Currently, FOR UPDATE may not 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 ] select_statement
select_statement is any SELECT statement without an ORDER BY, LIMIT,
or FOR UPDATE clause.
The INTERSECT operator computes the set intersection of the rows
returned by the involved SELECT statements. A row is in the intersec-
tion 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.
Multiple INTERSECT operators in the same SELECT statement are evalu-
ated 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).
EXCEPT CLAUSE
The EXCEPT clause has this general form:
select_statement EXCEPT [ ALL ] select_statement
select_statement is any SELECT statement without an ORDER BY, LIMIT,
or FOR UPDATE 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.
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.
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. Using the clause AS output_name, another name can be specified
for an output column. This name is primarily used to label the column
for display. It can also 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, one can
write table_name.* as a shorthand for the columns coming from just
that table.
ORDER BY CLAUSE
The optional ORDER BY clause has this general form:
ORDER BY expression [ ASC | DESC | USING operator ] [, ...]
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 ORDER BY clause causes the result rows to be sorted according to
the specified expressions. If two rows are equal according to the
leftmost expression, the 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.
The ordinal number refers to the ordinal (left-to-right) position of
the result column. This feature makes it possible to define an order-
ing 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 a result 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 result
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 may only specify an
output column name or number, not an expression.
If an ORDER BY expression is a simple name that matches both a result
column name and an input column name, ORDER BY will interpret it as
the result 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 may add the key word ASC (ascending) or DESC (descend-
ing) after any expression in the ORDER BY clause. If not specified,
ASC is assumed by default. Alternatively, a specific ordering operator
name may be specified in the USING clause. ASC is usually equivalent
to USING < and DESC is usually equivalent to USING >. (But the cre-
ator of a user-defined data type can define exactly what the default
sort ordering is, and it might correspond to operators with other
names.)
The null value sorts higher than any other value. In other words, with
ascending sort order, null values sort at the end, and with descending
sort order, null values sort at the beginning.
Character-string data is sorted according to the locale-specific col-
lation order that was established when the database cluster was ini-
tialized.
LIMIT CLAUSE
The LIMIT clause consists of two independent sub-clauses:
LIMIT { count | ALL }
OFFSET start
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.
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 may 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 differ-
ent 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 pre-
dictable 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.
DISTINCT CLAUSE
If DISTINCT is specified, all duplicate rows are removed from the
result set (one row is kept from each group of duplicates). ALL speci-
fies the opposite: all rows are kept; that is the default.
DISTINCT ON ( expression [, ...] ) keeps only the first row of each
set of rows where the given expressions evaluate to equal. The DIS-
TINCT ON expressions are interpreted using the same rules as for ORDER
BY (see above). Note that the ‘‘first row’’ of each set is unpre-
dictable 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 expres-
sion(s). The ORDER BY clause will normally contain additional expres-
sion(s) that determine the desired precedence of rows within each DIS-
TINCT ON group.
FOR UPDATE CLAUSE
The FOR UPDATE clause has this form:
FOR UPDATE [ OF table_name [, ...] ]
FOR UPDATE causes the rows retrieved by the SELECT statement to be
locked as though for update. This prevents them from being modified or
deleted by other transactions until the current transaction ends. That
is, other transactions that attempt UPDATE, DELETE, or SELECT FOR
UPDATE of these rows will be blocked until the current transaction
ends. Also, if an UPDATE, DELETE, or SELECT FOR UPDATE from another
transaction has already locked a selected row or rows, SELECT FOR
UPDATE will wait for the other transaction to complete, and will then
lock and return the updated row (or no row, if the row was deleted).
For further discussion see [XRef to MVCC].
If specific tables are named in FOR UPDATE, then only rows coming from
those tables are locked; any other tables used in the SELECT are sim-
ply read as usual.
FOR UPDATE cannot be used in contexts where returned rows can’t be
clearly identified with individual table rows; for example it can’t be
used with aggregation.
FOR UPDATE may appear before LIMIT for compatibility with PostgreSQL
versions before 7.3. It effectively executes after LIMIT, however, and
so that is the recommended place to write it.
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 individ-
ual 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 distribu-
tors 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;
SELECT * FROM distributors(111);
did | name
-----+-------------
111 | Walt Disney
CREATE FUNCTION distributors_2(int) RETURNS SETOF record AS ’
SELECT * FROM distributors WHERE did = $1;
SELECT * FROM distributors_2(111) AS (f1 int, f2 text);
f1 | f2
-----+-------------
111 | Walt Disney
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 straightfor-
ward 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.
A less obvious use is to abbreviate a normal SELECT from tables:
SELECT distributors.* WHERE distributors.name = ’Westward’;
did | name
-----+----------
108 | Westward
This works because an implicit FROM item is added for each table that
is referenced in other parts of the SELECT statement but not mentioned
in FROM.
While this is a convenient shorthand, it’s easy to misuse. For exam-
ple, the command
SELECT distributors.* FROM distributors d;
is probably a mistake; most likely the user meant
SELECT d.* FROM distributors d;
rather than the unconstrained join
SELECT distributors.* FROM distributors d, distributors distributors;
that he will actually get. To help detect this sort of mistake, Post-
greSQL will warn if the implicit-FROM feature is used in a SELECT
statement that also contains an explicit FROM clause. Also, it is pos-
sible to disable the implicit-FROM feature by setting the ADD_MISS-
ING_FROM parameter to false.
THE AS KEY WORD
In the SQL standard, the optional key word AS is just noise and can be
omitted without affecting the meaning. The PostgreSQL parser requires
this key word when renaming output columns because the type extensi-
bility features lead to parsing ambiguities without it. AS is
optional in FROM items, however.
NAMESPACE AVAILABLE TO GROUP BY AND ORDER BY
In the SQL92 standard, an ORDER BY clause may only use result column
names or numbers, while a GROUP BY clause may 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 inter-
pretation 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 result-
column names.
SQL99 uses a slightly different definition which is not entirely
upward compatible with SQL92. In most cases, however, PostgreSQL will
interpret an ORDER BY or GROUP BY expression the same way SQL99 does.
NONSTANDARD CLAUSES
The clauses DISTINCT ON, LIMIT, and OFFSET are not defined in the SQL
standard.
SQL - Language Statements 2008-01-03 SELECT()
