PostgreSQL 7.2 Documentation
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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 ] [, ...] ]
        [ FOR UPDATE [ OF tablename [, ...] ] ]
        [ LIMIT { count | ALL } ]
        [ OFFSET start ]

    where from_item can be:

    [ ONLY ] table_name [ * ]
        [ [ AS ] alias [ ( column_alias_list ) ] ]
    |
    ( select )
        [ AS ] alias [ ( column_alias_list ) ]
    |
    from_item [ NATURAL ] join_type from_item
        [ ON join_condition | USING ( join_column_list ) ]

### Inputs

`expression`

  The name of a table's column or an expression.

`output_name`

  Specifies another name for an output column using the AS clause. 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 the `output_name` cannot be used in the WHERE or HAVING clauses; write out the expression instead.

`from_item`

  A table reference, sub-SELECT, or JOIN clause. See below for details.

`condition`

  A boolean expression giving a result of true or false. See the WHERE and HAVING clause descriptions below.

`select`

  A select statement with all features except the ORDER BY, FOR UPDATE, and LIMIT clauses (even those can be used when the select is parenthesized).

FROM items can contain:

`table_name`

  The name 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.)

`alias`

  A substitute name for the preceding `table_name`. An alias is used for brevity or to eliminate ambiguity for self-joins (where the same table is scanned multiple times). 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.

`join_type`

  One of [ INNER ] JOIN, LEFT [ OUTER ] JOIN, RIGHT [ OUTER ] JOIN, FULL [ OUTER ] JOIN, or CROSS JOIN. For INNER and OUTER join types, exactly one of NATURAL, ON `join_condition`, or USING ( `join_column_list` ) must appear. For CROSS JOIN, none of these items may appear.

`join_condition`

  A qualification condition. This is similar to the WHERE condition except that it only applies to the two from_items being joined in this JOIN clause.

`join_column_list`

  A USING column list ( a, b, ... ) is shorthand for the ON condition left_table.a = right_table.a AND left_table.b = right_table.b ...

### Outputs

Rows

  The complete set of rows resulting from the query specification.

`count`

  The count of rows returned by the query.

## 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 WHERE Clause.)

Actually, the returned rows are not directly the rows produced by the FROM/WHERE/GROUP BY/HAVING clauses; rather, the output rows are formed by computing the SELECT output expressions for each selected row. * 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.

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. The DISTINCT ON expressions are interpreted using the same rules as for ORDER BY items; see below. 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 into groups of rows that match on one or more values. (See GROUP BY Clause.)

The HAVING clause allows selection of only those groups of rows meeting the specified condition. (See HAVING Clause.)

The ORDER BY clause causes the returned rows to be sorted in a specified order. If ORDER BY is not given, the rows are returned in whatever order the system finds cheapest to produce. (See ORDER BY Clause.)

SELECT queries can be combined using UNION, INTERSECT, and EXCEPT operators. Use parentheses if necessary to determine the ordering of these operators.

The UNION operator computes the collection of rows returned by the queries involved. Duplicate rows are eliminated unless ALL is specified. (See UNION Clause.)

The INTERSECT operator computes the rows that are common to both queries. Duplicate rows are eliminated unless ALL is specified. (See INTERSECT Clause.)

The EXCEPT operator computes the rows returned by the first query but not the second query. Duplicate rows are eliminated unless ALL is specified. (See EXCEPT Clause.)

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 LIMIT Clause.)

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

### FROM Clause

The FROM clause specifies one or more source tables for the SELECT. If multiple sources are specified, the result is conceptually the Cartesian product of all the rows in all the sources --- but usually qualification conditions are added to restrict the returned rows to a small subset of the Cartesian product.

When a FROM item is a simple table name, it implicitly includes rows from sub-tables (inheritance children) of the table. ONLY will suppress rows from sub-tables of the table. Before PostgreSQL 7.1, this was the default result, and adding sub-tables was done by appending * to the table name. This old behaviour is available via the command SET SQL_Inheritance TO OFF;

A FROM item can also be a parenthesized sub-SELECT (note that an alias clause is required for a sub-SELECT!). This is an extremely handy feature since it's the only way to get multiple levels of grouping, aggregation, or sorting in a single query.

Finally, a FROM item can be a JOIN clause, which combines two simpler FROM items. (Use parentheses if necessary to determine the order of nesting.)

A CROSS JOIN or INNER JOIN is a simple Cartesian product, the same as you get from listing the two items at the top level of FROM. 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 ON condition), plus one copy of each row in the left-hand table for which there was no right-hand row that passed the ON condition. This left-hand row is extended to the full width of the joined table by inserting NULLs for the right-hand columns. Note that only the JOIN's own ON or USING condition is considered while deciding which rows have matches. Outer ON or WHERE 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).

For all the JOIN types except CROSS JOIN, you must write exactly one of ON `join_condition`, USING ( `join_column_list` ), or NATURAL. ON is the most general case: you can write any qualification expression involving the two tables to be joined. A USING column list ( a, b, ... ) is shorthand for the ON condition 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 is shorthand for a USING list that mentions all similarly-named columns in the tables.

### 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. SELECT will ignore all rows for which the WHERE condition does not return TRUE.

### GROUP BY Clause

GROUP BY specifies a grouped table derived by the application of this clause:

    GROUP BY expression [, ...]

GROUP BY will condense into a single row all selected 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 for the SELECT output expression(s) to refer to ungrouped columns except within aggregate functions, since there would be more than one possible value to return for an ungrouped column.

A GROUP BY item can be an input column name, or the name or ordinal number of an output column (SELECT expression), or it can be 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.

### HAVING Clause

The optional HAVING condition has the general form:

    HAVING boolean_expr

where `boolean_expr` is the same as specified for the WHERE clause.

HAVING specifies a grouped table derived by the elimination of group rows that do not satisfy the `boolean_expr`. HAVING is different from WHERE: WHERE filters individual rows before application of GROUP BY, while HAVING filters group rows created by GROUP BY.

Each column referenced in `boolean_expr` shall unambiguously reference a grouping column, unless the reference appears within an aggregate function.

### ORDER BY Clause

    ORDER BY expression [ ASC | DESC | USING operator ] [, ...]

An ORDER BY item can be the name or ordinal number of an output column (SELECT expression), or it can be an arbitrary expression formed from input-column values. In case of ambiguity, an ORDER BY name will be interpreted as an output-column name.

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 proper name. This is never absolutely necessary because it is always possible to assign a name to a result column using the AS clause, e.g.:

    SELECT title, date_prod + 1 AS newlen FROM films ORDER BY newlen;

It is also possible to ORDER BY arbitrary expressions (an extension to SQL92), including fields that do not appear in the SELECT result list. Thus the following statement is legal:

    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 query may only specify an output column name or number, not an expression.

Note that if an ORDER BY item 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 mandated by the SQL92 standard.

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 >.

The null value sorts higher than any other value in a domain. In other words, with ascending sort order nulls sort at the end and with descending sort order nulls sort at the beginning.

### UNION Clause

    table_query UNION [ ALL ] table_query
        [ ORDER BY expression [ ASC | DESC | USING operator ] [, ...] ]
        [ LIMIT { count | ALL } ]
        [ OFFSET start ]

where `table_query` specifies any select expression without an ORDER BY, FOR UPDATE, or LIMIT clause. (ORDER BY and LIMIT can be attached to a sub-expression 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 collection (set union) of the 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.

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 the inputs of a UNION.

### INTERSECT Clause

    table_query INTERSECT [ ALL ] table_query
        [ ORDER BY expression [ ASC | DESC | USING operator ] [, ...] ]
        [ LIMIT { count | ALL } ]
        [ OFFSET start ]

where `table_query` specifies any select expression without an ORDER BY, FOR UPDATE, or LIMIT clause.

INTERSECT is similar to UNION, except that it produces only rows that appear in both query outputs, rather than rows that appear in either.

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 L and n duplicates in R will appear min(m,n) times.

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) unless otherwise specified by parentheses.

### EXCEPT Clause

    table_query EXCEPT [ ALL ] table_query
        [ ORDER BY expression [ ASC | DESC | USING operator ] [, ...] ]
        [ LIMIT { count | ALL } ]
        [ OFFSET start ]

where `table_query` specifies any select expression without an ORDER BY, FOR UPDATE, or LIMIT clause.

EXCEPT is similar to UNION, except that it produces only rows that appear in the left query's output but not in the right query's output.

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 L and n duplicates in R will appear max(m-n,0) times.

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.

### LIMIT Clause

        LIMIT { count | ALL }
        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 specify ORDER BY.

As of PostgreSQL 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 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.

## 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
    (17 rows)

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
    (5 rows)

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
    (2 rows)

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
    (13 rows)

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

### Extensions

PostgreSQL allows one to omit the FROM clause from a query. This feature was retained from the original PostQuel query language. It has a straightforward use to compute the results of simple constant expressions:

    SELECT 2+2;

     ?column?
    ----------
            4

Some other DBMSes cannot do this except by introducing a dummy one-row table to do the select from. A less obvious use is to abbreviate a normal select from one or more 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 the query but not mentioned in FROM. While this is a convenient shorthand, it's easy to misuse. For example, the query

    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, PostgreSQL 7.1 and later will warn if the implicit-FROM feature is used in a query that also contains an explicit FROM clause.

### SQL92

#### SELECT Clause

In the SQL92 standard, the optional keyword `AS` is just noise and can be omitted without affecting the meaning. The PostgreSQL parser requires this keyword when renaming output columns because the type extensibility features lead to parsing ambiguities in this context. `AS` is optional in FROM items, however.

The DISTINCT ON phrase is not part of SQL92. Nor are LIMIT and OFFSET.

In SQL92, an ORDER BY clause may only use result column names or numbers, while a GROUP BY clause may only use 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.

#### UNION/INTERSECT/EXCEPT Clause

The SQL92 syntax for UNION/INTERSECT/EXCEPT allows an additional CORRESPONDING BY option:

    table_query UNION [ALL]
        [CORRESPONDING [BY (column [,...])]]
        table_query

The CORRESPONDING BY clause is not supported by PostgreSQL.

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