MySQL Cookbook, 2nd Edition

Using a Join to Control Query Output Order

Problem

You want to sort a statement’s output using a characteristic of the output that cannot be specified using ORDER BY . For example, you want to sort a set of rows by subgroups, putting first those groups with the most rows and last those groups with the fewest rows. But “number of rows in each group” is not a property of individual rows, so you can’t use it for sorting.

Solution

Derive the ordering information and store it in an auxiliary table. Then join the original table to the auxiliary table, using the auxiliary table to control the sort order.

Discussion

Most of the time when you sort a query result, you use an ORDER BY clause that names which column or columns to use for sorting. But sometimes the values you want to sort by aren’t present in the rows to be sorted. This is the case when you want to use group characteristics to order the rows. The following example uses the rows in the driver_log table to illustrate this. The table contains these rows:

mysql>SELECT * FROM driver_log ORDER BY rec_id; +--------+-------+------------+-------+ | rec_id | name | trav_date | miles | +--------+-------+------------+-------+ | 1 | Ben | 2006-08-30 | 152 | | 2 | Suzi | 2006-08-29 | 391 | | 3 | Henry | 2006-08-29 | 300 | | 4 | Henry | 2006-08-27 | 96 | | 5 | Ben | 2006-08-29 | 131 | | 6 | Henry | 2006-08-26 | 115 | | 7 | Suzi | 2006-09-02 | 502 | | 8 | Henry | 2006-09-01 | 197 | | 9 | Ben | 2006-09-02 | 79 | | 10 | Henry | 2006-08-30 | 203 | ...