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 1.3). 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 2.2.1.3. 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.b, tbl1.c FROM ...
(see also Section 2.2.2).
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 retrieved 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 could be constant arithmetic expressions as well, for instance.
The entries in the select list can be assigned names for further processing. The "further processing" in this case is an optional sort specification and the client application (e.g., column headers for display). For example:
SELECT a AS value, b + c AS sum FROM ...
If no output column name is specified via AS, the system assigns a default 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.
Note: The naming of output columns here is different from that done in the FROM clause (see Section 2.2.1.3). This pipeline will in fact allow you to rename the same column twice, but the name chosen in the select list is the one that will be passed on.
After the select list has been processed, the result table may optionally be subject to the elimination of duplicates. The DISTINCT key word is written directly after the SELECT to enable this:
SELECT DISTINCT select_list ...
(Instead of DISTINCT the word ALL can be used to select the default behavior of retaining all rows.)
Obviously, two rows are considered distinct if they differ in at least one column value. NULLs 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 subselects in FROM the construct can be avoided, but it is very often the most convenient alternative.