SELECT
SQL - Language Statements
SELECT
Retrieve rows from a table or view.
1999-07-20
SELECT [ ALL | DISTINCT [ ON ( expression [, ...] ) ] ]
expression [ AS name ] [, ...]
[ INTO [ TEMPORARY | TEMP ] [ TABLE ] new_table ]
[ FROM table [ alias ] [, ...] ]
[ WHERE condition ]
[ GROUP BY column [, ...] ]
[ HAVING condition [, ...] ]
[ { UNION [ ALL ] | INTERSECT | EXCEPT } select ]
[ ORDER BY column [ ASC | DESC | USING operator ] [, ...] ]
[ FOR UPDATE [ OF class_name [, ...] ] ]
LIMIT { count | ALL } [ { OFFSET | , } start ]
1998-09-24
Inputs
expression
The name of a table's column or an expression.
name
Specifies another name for a column or an expression using
the AS clause. This name is primarily used to label the output
column. The name
cannot be used in the WHERE, GROUP BY, or HAVING clauses.
It can, however, be referenced in ORDER BY clauses.
TEMPORARY
TEMP
The table is created unique to this session, and is
automatically dropped on session exit.
new_table
If the INTO TABLE clause is specified, the result of the
query will be stored in another table with the indicated
name.
The target table (new_table) will
be created automatically and should not exist before this command.
Refer to SELECT INTO for more information.
The CREATE TABLE AS statement will also
create a new table from a select query.
table
The name of an existing table referenced by the FROM clause.
alias
An alternate name for the preceding
table.
It is used for brevity or to eliminate ambiguity for joins
within a single table.
condition
A boolean expression giving a result of true or false.
See the WHERE clause.
column
The name of a table's column.
select
A select statement with all features except the ORDER BY clause.
1998-09-24
Outputs
Rows
The complete set of rows resulting from the query specification.
count
The count of rows returned by the query.
1998-09-24
Description
SELECT will return rows from one or more tables.
Candidates for selection are rows which satisfy the WHERE condition;
if WHERE is omitted, all rows are candidates.
(See .)
DISTINCT will eliminate duplicate rows from the
result.
ALL (the default) will return all candidate rows,
including duplicates.
DISTINCT ON eliminates rows that match on all the
specified expressions, keeping only the first row of each set of
duplicates. 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 weatherReports
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 of unpredictable age
for each location.
The GROUP BY clause allows a user to divide a table
conceptually into groups.
(See .)
The HAVING clause specifies a grouped table derived by the
elimination of groups from the result of the previously
specified clause.
(See .)
The ORDER BY clause allows a user to specify that he/she
wishes the rows sorted according to the ASCending or
DESCending mode operator.
(See .)
The UNION operator allows the result to be the collection of rows
returned by the queries involved.
(See .)
The INTERSECT operator gives you the rows that are common to both queries.
(See .)
The EXCEPT operator gives you the rows returned by the first query but
not the second query.
(See .)
The FOR UPDATE clause allows the SELECT statement to perform
exclusive locking of selected rows.
The LIMIT clause allows a subset of the rows produced by the query
to be returned to the user.
(See .)
You must have SELECT privilege to a table to read its values
(See the GRANT/REVOKE statements).
1998-09-24
WHERE Clause
The optional WHERE condition has the general form:
WHERE boolean_expr
boolean_expr
can consist of any expression which evaluates to a boolean value.
In many cases, this expression will be
expr cond_op expr
or
log_op expr
where cond_op
can be one of: =, <, <=, >, >= or <>,
a conditional operator like ALL, ANY, IN, LIKE, or a
locally-defined operator,
and log_op can be one
of: AND, OR, NOT.
The comparison returns either TRUE or FALSE and all
instances will be discarded
if the expression evaluates to FALSE.
1998-09-24
GROUP BY Clause
GROUP BY specifies a grouped table derived by the application
of this clause:
GROUP BY column [, ...]
GROUP BY will condense into a single row all rows that share the
same values for the grouped columns. Aggregate functions, if any,
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 to refer to
ungrouped columns except within aggregate functions, since there
would be more than one possible value to return for an ungrouped column.
1998-09-24
HAVING Clause
The optional HAVING condition has the general form:
HAVING cond_expr
where cond_expr is the same
as specified for the WHERE clause.
HAVING specifies a grouped table derived by the elimination
of groups from the result of the previously specified clause
that do not meet the cond_expr.
Each column referenced in
cond_expr shall unambiguously
reference a grouping column, unless the reference appears within an
aggregate function.
1998-09-24
ORDER BY Clause
ORDER BY column [ ASC | DESC ] [, ...]
column can be either a column
name or an ordinal number.
The ordinal numbers refers to the ordinal (left-to-right) position
of the column. This feature makes it possible to define an ordering
on the basis of a column that does not have a proper name.
This is never absolutely necessary because it is always possible
to assign a name to a calculated column using the AS clause, e.g.:
SELECT title, date_prod + 1 AS newlen FROM films ORDER BY newlen;
From release 6.4 of PostgreSQL, it is also possible to ORDER BY
arbitrary expressions, including fields that do not appear in the
SELECT result list.
Thus the following statement is now legal:
SELECT name FROM distributors ORDER BY code;
Optionally one may add the keyword DESC (descending)
or ASC (ascending) after each column name in the ORDER BY clause.
If not specified, ASC is assumed by default. Alternatively, a
specific ordering operator name may be specified. ASC is equivalent
to USING '<' and DESC is equivalent to USING '>'.
1998-09-24
UNION Clause
table_query UNION [ ALL ] table_query
[ ORDER BY column [ ASC | DESC ] [, ...] ]
where
table_query
specifies any select expression without an ORDER BY clause.
The UNION operator allows the result to be the collection of rows
returned by the queries involved.
The two SELECTs that represent the direct operands of the UNION must
produce the same number of columns, and corresponding columns must be
of compatible data types.
By default, the result of UNION does not contain any duplicate rows
unless the ALL clause is specified.
Multiple UNION operators in the same SELECT statement are
evaluated left to right.
Note that the ALL keyword is not global in nature, being
applied only for the current pair of table results.
1998-09-24
INTERSECT Clause
table_query INTERSECT table_query
[ ORDER BY column [ ASC | DESC ] [, ...] ]
where
table_query
specifies any select expression without an ORDER BY clause.
The INTERSECT operator gives you the rows that are common to both queries.
The two SELECTs that represent the direct operands of the INTERSECT must
produce the same number of columns, and corresponding columns must be
of compatible data types.
Multiple INTERSECT operators in the same SELECT statement are
evaluated left to right, unless parentheses dictate otherwise.
1998-09-24
EXCEPT Clause
table_query EXCEPT table_query
[ ORDER BY column [ ASC | DESC ] [, ...] ]
where
table_query
specifies any select expression without an ORDER BY clause.
The EXCEPT operator gives you the rows returned by the first query but
not the second query.
The two SELECTs that represent the direct operands of the EXCEPT must
produce the same number of columns, and corresponding columns must be
of compatible data types.
Multiple EXCEPT operators in the same SELECT statement are
evaluated left to right, unless parentheses dictate otherwise.
2000-02-20
LIMIT Clause
LIMIT { count | ALL } [ { OFFSET | , } start ]
OFFSET start
where
count specifies the
maximum number of rows to return, and
start specifies the
number of rows to skip before starting to return rows.
LIMIT allows you to retrieve just a portion of the rows that are generated
by the rest of the query. If a limit count is given, no more than that
many rows will be returned. If an offset is given, that many rows will
be skipped before starting to return rows.
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 specified ORDER BY.
As of Postgres 7.0, the
query optimizer 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 give 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.
Usage
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
Une Femme est une Femme |102|Jean Luc Godard |1961-03-12|Romantic
Vertigo |103|Paramount |1958-11-14|Action
Becket |103|Paramount |1964-02-03|Drama
48 Hrs |103|Paramount |1982-10-22|Action
War and Peace |104|Mosfilm |1967-02-12|Drama
West Side Story |105|United Artists |1961-01-03|Musical
Bananas |105|United Artists |1971-07-13|Comedy
Yojimbo |106|Toho |1961-06-16|Drama
There's a Girl in my Soup|107|Columbia |1970-06-11|Comedy
Taxi Driver |107|Columbia |1975-05-15|Action
Absence of Malice |107|Columbia |1981-11-15|Action
Storia di una donna |108|Westward |1970-08-15|Romantic
The King and I |109|20th Century Fox|1956-08-11|Musical
Das Boot |110|Bavaria Atelier |1981-11-11|Drama
Bed Knobs and Broomsticks|111|Walt Disney | |Musical
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 hour';
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
This example shows how to obtain the union of the tables
distributors and
actors, restricting the results to those that begin
with letter W in each table. Only distinct rows are wanted, so the
ALL keyword 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
Compatibility
1998-09-24
Extensions
Postgres allows one to omit
the FROM clause from a query. This feature
was retained from the original PostQuel query language:
SELECT distributors.* WHERE name = 'Westwood';
did|name
---+----------------
108|Westward
1998-09-24
SQL92
1998-04-15
SELECT Clause
In the SQL92 standard, the optional keyword "AS"
is just noise and can be
omitted without affecting the meaning.
The Postgres parser requires this keyword when
renaming columns because the type extensibility features lead to
parsing ambiguities
in this context.
The DISTINCT ON phrase is not part of SQL92.
Nor are LIMIT and OFFSET.
1998-09-24
UNION Clause
The SQL92 syntax for UNION allows an
additional CORRESPONDING BY clause:
table_query UNION [ALL]
[CORRESPONDING [BY (column [,...])]]
table_query
The CORRESPONDING BY clause is not supported by
Postgres.