SQL/MySQL/Math/LOG

Use log

/*
mysql> select LOG(20);
+----------------+
| LOG(20)        |
+----------------+
| 2.995732273554 |
+----------------+
1 row in set (0.00 sec)
mysql> select LOG(20, 40);
+-----------------+
| LOG(20, 40)     |
+-----------------+
| 1.2313782131598 |
+-----------------+
1 row in set (0.01 sec)

*/  
select LOG(20);
select LOG(20, 40);

Use LOG10 and LOG2

/*
mysql> select LOG10(10);
+-----------+
| LOG10(10) |
+-----------+
|         1 |
+-----------+
1 row in set (0.00 sec)
mysql> select LOG2(8);
+---------+
| LOG2(8) |
+---------+
|       3 |
+---------+
1 row in set (0.00 sec)

*/  
select LOG10(10);
select LOG2(8);