Syntax:
SLEEP(duration)

Sleeps (pauses) for the number of seconds given by the duration
argument, then returns 0. The duration may have a fractional part. If
the argument is NULL or negative, SLEEP() produces a warning, or an
error in strict SQL mode.

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 is true
whether the query is killed or times out:

o This statement is interrupted using KILL QUERY from another session:

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

o This statement is interrupted by timing out:

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

When SLEEP() is only part of a query that is interrupted, the query
returns an error:

o 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

o This statement is interrupted by timing out:

mysql> SELECT /*+ MAX_EXECUTION_TIME(1000) */ 1 FROM t1 WHERE SLEEP(1000);
ERROR 3024 (HY000): Query execution was interrupted, maximum statement
execution time exceeded

URL: https://dev.mysql.com/doc/refman/8.4/en/miscellaneous-functions.html

