mysql> select * from t1;
+------+------+
| c    | d    |
+------+------+
| 0.10 |  0.1 |
| 0.20 |  0.2 |
+------+------+
2 rows in set (0,00 sec)

mysql> select sum(c) as decimal_, sum(d) as double_ from t1;
+----------+---------------------+
| decimal_ | double_             |
+----------+---------------------+
|     0.30 | 0.30000000000000004 |
+----------+---------------------+
1 row in set (0,00 sec)

mysql> desc t1;
+-------+--------------+------+-----+---------+-------+
| Field | Type         | Null | Key | Default | Extra |
+-------+--------------+------+-----+---------+-------+
| c     | decimal(2,2) | YES  |     | NULL    |       |
| d     | double       | YES  |     | NULL    |       |
+-------+--------------+------+-----+---------+-------+