PostgreSQL 8.2.14 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 ] [, ...] ]
        [ 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 below.)

2. If the `WHERE` clause is specified, all rows that do not satisfy the condition are eliminated from the output. (See WHERE Clause 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 and HAVING Clause below.)

4. The actual output rows are computed using the `SELECT` output expressions for each selected row. (See SELECT List 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, INTERSECT Clause, and EXCEPT Clause 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 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 below.)

8. If the `LIMIT` or `OFFSET` clause is specified, the `SELECT` statement only returns a subset of the result rows. (See LIMIT Clause below.)

9. If `FOR UPDATE` or `FOR SHARE` is specified, the `SELECT` statement locks the selected rows against concurrent updates. (See FOR UPDATE/FOR SHARE Clause 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. A VALUES command can also be used here.

`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, `JOIN`s 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 Chapter 12.

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 Chapter 12). You can use the `NOWAIT` option of LOCK 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`.

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. A `FOR UPDATE` or `FOR SHARE` clause without a table list affects all tables used in the command. If `FOR UPDATE` or `FOR SHARE` is applied to a view or sub-query, it affects all tables used in the view or sub-query.

Multiple `FOR UPDATE` and `FOR SHARE` clauses can be written if it is necessary to specify different locking behavior for different tables. If the same table is mentioned (or implicitly affected) by both `FOR UPDATE` and `FOR SHARE` clauses, then it is processed as `FOR UPDATE`. Similarly, a table is processed as `NOWAIT` if that is specified in any of the clauses affecting it.

`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.

Caution
Avoid locking a row and then modifying it within a later savepoint or PL/pgSQL exception block. A subsequent rollback would cause the lock to be lost. For example,
    BEGIN;
    SELECT * FROM mytable WHERE key = 1 FOR UPDATE;
    SAVEPOINT s;
    UPDATE mytable SET ... WHERE key = 1;
    ROLLBACK TO s;
After the `ROLLBACK`, the row is effectively unlocked, rather than returned to its pre-savepoint state of being locked but not modified. This hazard occurs if a row locked in the current transaction is updated or deleted, or if a shared lock is upgraded to exclusive: in all these cases, the former lock state is forgotten. If the transaction is then rolled back to a state between the original locking command and the subsequent change, the row will appear not to be locked at all. This is an implementation deficiency which will be addressed in a future release of PostgreSQL.

Caution
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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