MySQL Tutorial/Math Numeric Functions/SQRT

SQRT(-16);

mysql>
mysql> SELECT SQRT(-16);
+-----------+
| SQRT(-16) |
+-----------+
|      NULL |
+-----------+
1 row in set (0.00 sec)
mysql>

SQRT(X) returns the square root of a non-negative number X.

mysql>
mysql> SELECT SQRT(4);
+---------+
| SQRT(4) |
+---------+
|       2 |
+---------+
1 row in set (0.00 sec)
mysql>
mysql> SELECT SQRT(20);
+-----------------+
| SQRT(20)        |
+-----------------+
| 4.4721359549996 |
+-----------------+
1 row in set (0.00 sec)
mysql>