Re: Best way to store numeric data?

[Roger Baklund]

Michael Dykman wrote:
[...]
The MySQL implementation also supports this optional
precision specification, but the precision value is used only to
determine storage size.
Right. This means you can not have 15 decimals precision using DOUBLE:
mysql> use test
Database changed
mysql> create table dtest(d double(18,15));
Query OK, 0 rows affected (0.01 sec)
mysql> insert into dtest values (6.984789027653891),(39.484789039053891);
Query OK, 2 rows affected (0.02 sec)
Records: 2  Duplicates: 0  Warnings: 0
mysql> select * from dtest;
+--------------------+
| d                  |
+--------------------+
|  6.984789027653892 |
| 39.484789039053894 |
+--------------------+
2 rows in set (0.00 sec)
Last digit is "wrong" in both test rows. Increasing precision does not help:
mysql> create table d2test(d double(18,16));
Query OK, 0 rows affected (0.02 sec)
mysql> insert into d2test values (6.984789027653891),(39.484789039053891);
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0
mysql> select * from d2test;
+---------------------+
| d                   |
+---------------------+
|  6.9847890276538909 |
| 39.4847890390538940 |
+---------------------+
2 rows in set (0.00 sec)
This is no error, it is the approximate data type at work... it simply 
can not store the exact value.

--
Roger
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