7.3. Select Lists
As shown in the previous section,
the table expression in the
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.
7.3.1. Select-List Items
The simplest kind of select list is
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
are either the actual names of the columns of tables referenced
FROM clause, or the aliases given to them as
explained in Section 188.8.131.52. 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
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 Section 8.16.5 for more about
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
they can be constant arithmetic expressions, for instance.
7.3.2. Column Labels
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
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.
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
AS or double-quote the output column name.
The naming of output columns here is different from that done in
FROM clause (see Section 184.108.40.206). 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:
DISTINCT the key word
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 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 ON processing occurs after
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
construct can be avoided, but it is often the most convenient