SELECT

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 Postgres 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
    

     expr cond_op expr
    

     log_op expr
    

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
    

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;
    

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 >.

UNION Clause

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

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 ]]
    

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 ]]
    

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 ]
    OFFSET start
    

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.

Extensions

Postgres 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
  

SELECT distributors.* WHERE name = 'Westward';

 did | name
-----+----------
 108 | Westward
  

SELECT distributors.* FROM distributors d;
  

SELECT d.* FROM distributors d;
  

SELECT distributors.* FROM distributors d, distributors distributors;
  

SQL92

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