|PostgreSQL 9.0.23 Documentation|
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As shown in the previous section, the table expression in the SELECT command constructs an intermediate virtual table by possibly combining tables, views, eliminating rows, grouping, etc. This table is finally passed on to processing by the select list. The select list determines which columns of the intermediate table are actually output.
The simplest kind of select list is * which emits all columns that the table expression produces. Otherwise, a select list is a comma-separated list of value expressions (as defined in Section 4.2). For instance, it could be a list of column names:
SELECT a, b, c FROM ...
The columns names a, b, and c are either the actual names of the columns of tables referenced in the FROM clause, or the aliases given to them as explained in Section 184.108.40.206. The name space available in the select list is the same as in the WHERE clause, unless grouping is used, in which case it is the same as in the HAVING clause.
If more than one table has a column of the same name, the table name must also be given, as in:
SELECT tbl1.a, tbl2.a, tbl1.b FROM ...
When working with multiple tables, it can also be useful to ask for all the columns of a particular table:
SELECT tbl1.*, tbl2.a FROM ...
(See also Section 7.2.2.)
If an arbitrary value expression is used in the select list, it conceptually adds a new virtual column to the returned table. The value expression is evaluated once for each result row, with the row's values substituted for any column references. But the expressions in the select list do not have to reference any columns in the table expression of the FROM clause; they can be constant arithmetic expressions, for instance.
The entries in the select list can be assigned names for subsequent processing, such as for use in an ORDER BY clause or for display by the client application. For example:
SELECT a AS value, b + c AS sum FROM ...
If no output column name is specified using AS, the system assigns a default column name. For simple column references, this is the name of the referenced column. For function calls, this is the name of the function. For complex expressions, the system will generate a generic name.
The AS keyword is optional, but only if the new column name does not match any PostgreSQL keyword (see Appendix C). To avoid an accidental match to a keyword, you can double-quote the column name. For example, VALUE is a keyword, so this does not work:
SELECT a value, b + c AS sum FROM ...
but this does:
SELECT a "value", b + c AS sum FROM ...
For protection against possible future keyword additions, it is recommended that you always either write AS or double-quote the output column name.
Note: The naming of output columns here is different from that done in the FROM clause (see Section 220.127.116.11). It is possible to rename the same column twice, but the name assigned in the select list is the one that will be passed on.
After the select list has been processed, the result table can optionally be subject to the elimination of duplicate rows. The DISTINCT key word is written directly after SELECT to specify this:
SELECT DISTINCT select_list ...
(Instead of DISTINCT the key word ALL can be used to specify the default behavior of retaining all rows.)
Obviously, two rows are considered distinct if they differ in at least one column value. Null values are considered equal in this comparison.
Alternatively, an arbitrary expression can determine what rows are to be considered distinct:
SELECT DISTINCT ON (expression [, expression ...]) select_list ...
Here expression is an arbitrary value expression that is evaluated for all rows. A set of rows for which all the expressions are equal are considered duplicates, and only the first row of the set is kept in the output. Note that the "first row" of a set is unpredictable unless the query is sorted on enough columns to guarantee a unique ordering of the rows arriving at the DISTINCT filter. (DISTINCT ON processing occurs after ORDER BY sorting.)
The DISTINCT ON clause is not part of the SQL standard and is sometimes considered bad style because of the potentially indeterminate nature of its results. With judicious use of GROUP BY and subqueries in FROM, this construct can be avoided, but it is often the most convenient alternative.