Finding Smallest or Largest Summary Values

7.15.1 Problem

You want to compute per-group summary values, but display only the smallest or largest of them.

7.15.2 Solution

Add a LIMIT clause to the query.

7.15.3 Discussion

MIN( ) and MAX( ) find the values at the endpoints of a range of values, but if you want to know the extremes of a set of summary values, those functions won't work. The arguments to MIN( ) and MAX( ) cannot be other aggregate functions. For example, you can easily find per-driver mileage totals:

mysql> SELECT name, SUM(miles)
 -> FROM driver_log
 -> GROUP BY name;
+-------+------------+
| name | SUM(miles) |
+-------+------------+
| Ben | 362 |
| Henry | 911 |
| Suzi | 893 |
+-------+------------+

But to select only the record for the driver with the most miles, this doesn't work:

mysql> SELECT name, SUM(miles)
 -> FROM driver_log
 -> GROUP BY name
 -> HAVING SUM(miles) = MAX(SUM(name));
ERROR 1111 at line 1: Invalid use of group function

Instead, order the rows with the largest SUM( ) values first and use LIMIT to select the first record:

mysql> SELECT name, SUM(miles) AS 'total miles'
 -> FROM driver_log
 -> GROUP BY name
 -> ORDER BY 'total miles' DESC LIMIT 1;
+-------+-------------+
| name | total miles |
+-------+-------------+
| Henry | 911 |
+-------+-------------+

An alias is used in the ORDER BY clause because ORDER BY cannot refer directly to aggregate functions, as discussed earlier in Recipe 7.14.

Note that if there is more than one row with the given summary value, this type of query won't tell you that. For example, you might attempt to ascertain the most common initial letter for state names like this:

mysql> SELECT LEFT(name,1) AS letter, COUNT(*) AS count FROM states
 -> GROUP BY letter ORDER BY count DESC LIMIT 1;
+--------+-------+
| letter | count |
+--------+-------+
| M | 8 |
+--------+-------+

But eight state names also begin with N. If you need to know all most-frequent values when there may be more than one of them, a two-query approach will be more useful:

mysql> SELECT LEFT(name,1) AS letter, @max:=COUNT(*) AS count FROM states
 -> GROUP BY letter ORDER BY count DESC LIMIT 1;
mysql> SELECT LEFT(name,1) AS letter, COUNT(*) AS count FROM states
 -> GROUP BY letter HAVING count = @max;
+--------+-------+
| letter | count |
+--------+-------+
| M | 8 |
| N | 8 |
+--------+-------+