SELECT (l)

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 select(l).)

DISTINCT will eliminate all duplicate rows from the selection. DISTINCT ON column will eliminate all duplicates in the specified column; this is equivalent to using GROUP BY column. ALL will return all candidate rows, including duplicates.

The GROUP BY clause allows a user to divide a table conceptually into groups. (See select(l).)

The HAVING clause specifies a grouped table derived by the elimination of groups from the result of the previously specified clause. (See select(l).)

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 select(l).)

The UNION clause allows the result to be the collection of rows returned by the queries involved. (See select(l).)

The INTERSECT give you the rows that are common to both queries. (See select(l).)

The EXCEPT give you the rows in the upper query not in the lower query. (See select(l).)

The FOR UPDATE clause allows the SELECT statement to perform exclusive locking of selected rows.

The LIMIT clause allows control over which rows are returned by the query.

You must have SELECT privilege to a table to read its values (See the GRANT/REVOKE statements).

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.

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; aggregates return values derived from all rows that make up the group. The value returned for an ungrouped and unaggregated column is dependent on the order in which rows happen to be read from the database.

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.

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 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, the columns in the ORDER BY clause do not need to appear in the SELECT clause. 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.

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 clause allows the result to be the collection of rows returned by the queries involved. (See UNION clause). The two tables that represent the direct operands of the UNION must have 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.

table_query INTERSECT table_query
    [ ORDER BY column [ ASC | DESC ] [, ...] ]

where table_query specifies any select expression without an ORDER BY clause.

The INTERSECT clause allows the result to be all rows that are common to the involved queries. The two tables that represent the direct operands of the INTERSECT must have 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.

table_query EXCEPT table_query
     [ ORDER BY column [ ASC | DESC ] [, ...] ]

where table_query specifies any select expression without an ORDER BY clause.

The EXCEPT clause allows the result to be rows from the upper query that are not in the lower query. (See EXCEPT clause). The two tables that represent the direct operands of the EXCEPT must have 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.

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 to be used, 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

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

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.

In the SQL92 standard, the new column name specified in an "AS" clause may be referenced in GROUP BY and HAVING clauses. This is not currently allowed in Postgres.

The DISTINCT ON phrase is not part of SQL92.

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.