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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 | SHARE } [ OF table_name [, ...] ] [ NOWAIT ] ]
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 zero 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(5)] 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(5)] 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(5)] and HAVING Clause [select(5)] below.)
4. The actual output rows are computed using the SELECT output
expressions for each selected row. (See SELECT List [select(5)]
below.)
5. 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(5)], INTERSECT Clause [select(l)], and EXCEPT Clause
[select(5)] 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(5)] 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(5)] below.)
8. If the LIMIT or OFFSET clause is specified, the SELECT statement only
returns a subset of the result rows. (See LIMIT Clause [select(5)]
below.)
9. If the FOR UPDATE or FOR SHARE clause is specified, the SELECT
statement locks the selected rows against concurrent updates. (See FOR
UPDATE/FOR SHARE Clause [select(5)] below.)
You must have SELECT privilege on a table to read its values. The use of
FOR UPDATE or FOR SHARE 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.
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, 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 indicate that
descendant tables are to be scanned, but in the current 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 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.
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 especially
useful for functions that return result sets, but any function can be
used.) This acts as though its output were created as a temporary
table for the duration of this single SELECT command. An alias may
also be used. 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. 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 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
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 commas
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 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 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 equivalent 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 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 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-column 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 computed 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 reference a grouping
column, unless the reference appears within an aggregate function.
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.
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.
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, FOR
UPDATE, or FOR 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.)
Multiple UNION operators in the same SELECT statement are evaluated left to
right, unless otherwise indicated by parentheses.
Currently, FOR UPDATE and FOR SHARE 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, FOR
UPDATE, or FOR 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.
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 UPDATE and FOR SHARE may not 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 ] select_statement
select_statement is any SELECT statement without an ORDER BY, LIMIT, FOR
UPDATE, or FOR 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.
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 UPDATE and FOR SHARE may not 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 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 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 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 (descending)
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 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.)
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 collation
order that was established when the database cluster was initialized.
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 specifies 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 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.
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 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.
FOR UPDATE/FOR SHARE CLAUSE
The FOR UPDATE clause has this form:
FOR UPDATE [ OF table_name [, ...] ] [ NOWAIT ]
The closely related FOR SHARE clause has this form:
FOR SHARE [ OF table_name [, ...] ] [ NOWAIT ]
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 the
documentation.
To prevent the operation from waiting for other transactions to commit, use
the NOWAIT option. SELECT FOR UPDATE NOWAIT reports an error, rather than
waiting, if a selected row cannot be locked immediately. Note that NOWAIT
applies only to the row-level lock(s) - the required ROW SHARE table-level
lock is still taken in the ordinary way (see the documentation). You can
use the NOWAIT option of LOCK [lock(5)] if you need to acquire the table-
level lock without waiting.
FOR SHARE behaves similarly, except that it acquires a shared rather than
exclusive lock on each retrieved row. A shared lock blocks other
transactions from performing UPDATE, DELETE, or SELECT FOR UPDATE on these
rows, but it does not prevent them from performing SELECT FOR SHARE.
It is currently not allowed for a single SELECT statement to include both
FOR UPDATE and FOR SHARE, nor can different parts of the statement use both
NOWAIT and normal waiting mode.
If specific tables are named in FOR UPDATE or FOR SHARE, then only rows
coming from those tables are locked; any other tables used in the SELECT
are simply read as usual.
FOR UPDATE and FOR SHARE cannot be used in contexts where returned rows
can't be clearly identified with individual table rows; for example they
can't be used with aggregation.
It is possible for a SELECT command using both LIMIT and FOR UPDATE/SHARE
clauses to return fewer rows than specified by LIMIT. This is because
LIMIT is applied first. The command selects the specified number of rows,
but might then block trying to obtain lock on one or more of them. Once
the SELECT unblocks, the row might have been deleted or updated so that it
does not meet the query WHERE condition anymore, in which case it will not
be returned.
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
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 the default behavior, because it does not
comply with the SQL standard, and is considered by many to be error-prone.
For compatibility with applications that rely on this behavior the
add_missing_from configuration variable can be enabled.
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 extensibility
features lead to parsing ambiguities without it. AS is optional in FROM
items, however.
NAMESPACE AVAILABLE TO GROUP BY AND ORDER BY
In the SQL-92 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 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 result-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.
NONSTANDARD CLAUSES
The clauses DISTINCT ON, LIMIT, and OFFSET are not defined in the SQL
standard.
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