MySQL Reference Manual :: 12.10.3 GROUP BY with Hidden Fields

MySQL extends the use of GROUP BY so that you can use columns or calculations in the SELECT list that don't appear in the GROUP BY clause. This stands for any possible value for this group. You can use this to get better performance by avoiding sorting and grouping on unnecessary items. For example, you don't need to group on customer.name in the following query:

mysql> SELECT order.custid, customer.name, MAX(payments)
    ->        FROM order,customer
    ->        WHERE order.custid = customer.custid
    ->        GROUP BY order.custid;

In standard SQL, you would have to add customer.name to the GROUP BY clause. In MySQL, the name is redundant if you don't run in ANSI mode.

Do not use this feature if the columns you omit from the GROUP BY part are not unique in the group! You get unpredictable results.

In some cases, you can use MIN() and MAX() to obtain a specific column value even if it isn't unique. The following gives the value of column from the row containing the smallest value in the sort column:

SUBSTR(MIN(CONCAT(RPAD(sort,6,' '),column)),7)

See Section 3.6.4, “The Rows Holding the Group-wise Maximum of a Certain Field”.

Note that if you are using MySQL 3.22 (or earlier) or if you are trying to follow standard SQL, you can't use expressions in GROUP BY or ORDER BY clauses. You can work around this limitation by using an alias for the expression:

mysql> SELECT id,FLOOR(value/100) AS val FROM tbl_name
    ->     GROUP BY id, val ORDER BY val;

In MySQL 3.23 and up, aliases are unnecessary. You can use expressions in GROUP BY and ORDER BY clauses. For example:

mysql> SELECT id, FLOOR(value/100) FROM tbl_name ORDER BY RAND();