Syntax:
SLEEP(duration)

Sleeps (pauses) for the number of seconds given by the duration
argument, then returns 0. If SLEEP() is interrupted, it returns 1. The
duration may have a fractional part.

When sleep returns normally (without interruption), it returns 0:

mysql> SELECT SLEEP(1000);
+-------------+
| SLEEP(1000) |
+-------------+
|           0 |
+-------------+

When SLEEP() is the only thing invoked by a query that is interrupted,
it returns 1 and the query itself returns no error. This statement is
interrupted using KILL QUERY from another session:

mysql> SELECT SLEEP(1000);
+-------------+
| SLEEP(1000) |
+-------------+
|           1 |
+-------------+

When SLEEP() is only part of a query that is interrupted, the query
returns an error. This statement is interrupted using KILL QUERY from
another session:

mysql> SELECT 1 FROM t1 WHERE SLEEP(1000);
ERROR 1317 (70100): Query execution was interrupted

URL: http://dev.mysql.com/doc/refman/5.5/en/miscellaneous-functions.html

