ORDER BY clause

This topic describes the ORDER BY clause and how to use this clause.

The optional ORDERBY clause has the following syntax:

ORDER BY expression [ ASC | DESC ] [, ...]

expression can be the name or the ordinal number of an output column in the SELECT list. It can also be an arbitrary expression that is formed from input-column values.

The ORDER BY clause enables the result rows to be sorted based on the specified expressions. If two rows are equal based on the leftmost expression, they are compared based on the next expression. If they are equal based on all the 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 defines a sorting by using a column that does not have a unique name. This is not necessary because you can use the AS clause to assign a name to a result column.

You can also use an expression in the ORDERBY clause, including columns that do not appear in the SELECT result list. Therefore, the following statement is valid:

SELECT ename FROM emp ORDER BY empno;

An ORDER BY clause applied to the result of a UNION, INTERSECT, or MINUS clause can specify only an output column name or number rather than an expression. This is the limit of that feature.

If an ORDER BY expression is a simple name that matches both output and input column names, ORDER BY interprets the simple name as the output column name. This is the opposite of the choice that is made by the GROUP BY clause in the same situation. This inconsistency is made to be compatible with the SQL standard.

You can add the ASC (ascending) or DESC (descending) keyword after an expression in the ORDER BY clause. If you do not specify ASC or DESC, ASC is used by default.

The null value is sorted in a higher order than other values. In other words, null values are at the end of an ascending order and are at the beginning of a descending order.

String data is sorted by using the collation for the specific locale. The collation is created when the database cluster is initialized.

SELECT * FROM dept ORDER BY dname;

 deptno |   dname    |   loc
--------+------------+----------
     10 | ACCOUNTING | NEW YORK
     40 | OPERATIONS | BOSTON
     20 | RESEARCH   | DALLAS
     30 | SALES      | CHICAGO
(4 rows)

SELECT * FROM dept ORDER BY 2;

 deptno |   dname    |   loc
--------+------------+----------
     10 | ACCOUNTING | NEW YORK
     40 | OPERATIONS | BOSTON
     20 | RESEARCH   | DALLAS
     30 | SALES      | CHICAGO
(4 rows)