MySQL Cookbook, 2nd Edition

Choosing Appropriate LIMIT Values

Problem

LIMITdoesn’t seem to do what you want it to.

Solution

Be sure that you understand what question you’re asking. It may be that LIMIT is exposing some interesting subtleties in your data that you have not considered.

Discussion

LIMITn is useful in conjunction with ORDERBY for selecting smallest or largest values from a result set. But does that actually give you the rows with the n smallest or largest values? Not necessarily! It does if your rows contain unique values, but not if there are duplicates. You may find it necessary to run a preliminary query first to help you choose the proper LIMIT value.

To see why this is, consider the following dataset, which shows the American League pitchers who won 15 or more games during the 2001 baseball season (you can find this data in the al_winner.sql file in the tables directory of the recipes distribution):

mysql>SELECT name, wins FROM al_winner
    -> ORDER BY wins DESC, name;
+----------------+------+
| name           | wins |
+----------------+------+
| Mulder, Mark   |   21 |
| Clemens, Roger |   20 |
| Moyer, Jamie   |   20 |
| Garcia, Freddy |   18 |
| Hudson, Tim    |   18 |
| Abbott, Paul   |   17 |
| Mays, Joe      |   17 |
| Mussina, Mike  |   17 |
| Sabathia, C.C. |   17 |
| Zito, Barry    |   17 |
| Buehrle, Mark  |   16 |
| Milton, Eric   |   15 |
| Pettitte, Andy |   15 |
| Radke, Brad    |   15 |
| Sele, Aaron    |   15 |
+----------------+------+

If you want to know who won the most games, adding LIMIT 1 to the preceding statement gives you the correct answer ...