LN(X) returns the natural logarithm of X; that is, the base-e logarithm of X : LN « Math Numeric Functions « MySQL Tutorial

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

mysql>
mysql> SELECT LN(2);
+------------------+
| LN(2)            |
+------------------+
| 0.69314718055995 |
+------------------+
1 row in set (0.00 sec)

mysql>
mysql> SELECT LN(-2);
+--------+
| LN(-2) |
+--------+
|   NULL |
+--------+
1 row in set (0.00 sec)

mysql>