SIGN(X) returns the sign of the argument as -1, 0, or 1, depending on whether X is negative, zero, or positive : SIGN « Math Numeric Functions « MySQL Tutorial

1. MySQL Tutorial
2. Math Numeric Functions
3. SIGN

mysql>
mysql> SELECT SIGN(-32);
+-----------+
| SIGN(-32) |
+-----------+
|        -1 |
+-----------+
1 row in set (0.00 sec)

mysql>
mysql> SELECT SIGN(0);
+---------+
| SIGN(0) |
+---------+
|       0 |
+---------+
1 row in set (0.00 sec)

mysql>
mysql> SELECT SIGN(234);
+-----------+
| SIGN(234) |
+-----------+
|         1 |
+-----------+
1 row in set (0.00 sec)

mysql>