The example from MySQL documentation http://dev.mysql.com/doc/refman/5.1/en/problems-with-float.html (i shrink it, documentation for this section is the same for 5.5)
mysql> create table t1 (i int, d1 double, d2 double);
mysql> insert into t1 values (2, 0.00 , 0.00),
(2, -13.20, 0.00),
(2, 59.60 , 46.40),
(2, 30.40 , 30.40);
mysql> select
i,
sum(d1) as a,
sum(d2) as b
from
t1
group by
i
having a <> b; -- a != b
+------+-------------------+------+
| i | a | b |
+------+-------------------+------+
| 2 | 76.80000000000001 | 76.8 |
+------+-------------------+------+
1 row in set (0.00 sec)
Basically if you sum a you get 0-13.2+59.6+30.4 = 76.8. If we sum up b we get 0+0+46.4+30.4=76.8. The sum of a and b is the same but MySQL documentation says:
A floating-point value as written in an SQL statement may not be the same as the value represented internally.
If we repeat the same with decimal:
mysql> create table t2 (i int, d1 decimal(60,30), d2 decimal(60,30));
Query OK, 0 rows affected (0.09 sec)
mysql> insert into t2 values (2, 0.00 , 0.00),
(2, -13.20, 0.00),
(2, 59.60 , 46.40),
(2, 30.40 , 30.40);
Query OK, 4 rows affected (0.07 sec)
Records: 4 Duplicates: 0 Warnings: 0
mysql> select
i,
sum(d1) as a,
sum(d2) as b
from
t2
group by
i
having a <> b;
Empty set (0.00 sec)
The result as expected is empty set.
So as long you do not perform any SQL arithemetic operations you can use DOUBLE, but I would still prefer DECIMAL.
Another thing to note about DECIMAL is rounding if fractional part is too large. Example:
mysql> create table t3 (d decimal(5,2));
Query OK, 0 rows affected (0.07 sec)
mysql> insert into t3 (d) values(34.432);
Query OK, 1 row affected, 1 warning (0.10 sec)
mysql> show warnings;
+-------+------+----------------------------------------+
| Level | Code | Message |
+-------+------+----------------------------------------+
| Note | 1265 | Data truncated for column 'd' at row 1 |
+-------+------+----------------------------------------+
1 row in set (0.00 sec)
mysql> select * from t3;
+-------+
| d |
+-------+
| 34.43 |
+-------+
1 row in set (0.00 sec)