MySQL Cookbook

Sorting Expression Results

Problem

You want to sort a query result based on values calculated from a column, rather than using the values actually stored in the column.

Solution

Put the expression that calculates the values in the ORDER BY clause. For older versions of MySQL that don’t support ORDER BY expressions, use a workaround.

Discussion

One of the columns in the mail table shows how large each mail message is, in bytes:

mysql> SELECT * FROM mail;
+---------------------+---------+---------+---------+---------+---------+
| t                   | srcuser | srchost | dstuser | dsthost | size    |
+---------------------+---------+---------+---------+---------+---------+
| 2001-05-11 10:15:08 | barb    | saturn  | tricia  | mars    |   58274 |
| 2001-05-12 12:48:13 | tricia  | mars    | gene    | venus   |  194925 |
| 2001-05-12 15:02:49 | phil    | mars    | phil    | saturn  |    1048 |
| 2001-05-13 13:59:18 | barb    | saturn  | tricia  | venus   |     271 |
...

Suppose you want to retrieve records for “big” mail messages (defined as those larger than 50,000 bytes), but you want them to be displayed and sorted by sizes in terms of kilobytes, not bytes. In this case, the values to sort are calculated by an expression. You can use ORDER BY to sort expression results, although the way you write the query may depend on your version of MySQL.

Prior to MySQL 3.23.2, expressions in ORDER BY clauses are not allowed. To work around this problem, specify the expression in the output column list and either refer to it by position or give it an alias and ...