/*
SIGN function returns the sign of the supplied value X as -1, 0, or 1,
depending on whether the value X is negative, zero, or positive, respectively.
*/
select SIGN(0);
select SIGN(-3);
select SIGN(3);
/*
mysql> select SIGN(0);
+---------+
| SIGN(0) |
+---------+
| 0 |
+---------+
1 row in set (0.00 sec)
mysql> select SIGN(-3);
+----------+
| SIGN(-3) |
+----------+
| -1 |
+----------+
1 row in set (0.00 sec)
mysql> select SIGN(3);
+---------+
| SIGN(3) |
+---------+
| 1 |
+---------+
1 row in set (0.00 sec)
*/
