1.float類型
float列類型默認長度查不到結果，必須指定精度，
比如 num  float,  insert into  table (num) values (0.12); select  * from table where num=0.12的話，empty set。
num float(9,7),  insert into  table (num) values (0.12); select  * from table where num=0.12的話會查到這條記錄。

mysql> create table tt
    -> (
    -> num  float(9,3)
    -> );
Query OK, 0 rows affected (0.03 sec)

mysql> insert into tt(num)values(1234567.8);
ERROR 1264 (22003): Out of range value for column 'num' at row 1
注：超出字段范圍，無法插入

mysql> insert into tt(num)values(123456.8);
Query OK, 1 row affected (0.00 sec)

mysql> select  * from  tt;
+------------+
| num        |
+------------+
| 123456.797 |
+------------+
1 row in set (0.00 sec)
注：小數位數不夠，自動補齊，但是存在一個問題就是如上的近似值。

mysql> insert into tt(num)values(123456.867);
Query OK, 1 row affected (0.04 sec)

mysql> select * from   tt;
+------------+
| num        |
+------------+
| 123456.797 |
| 123456.797 |
| 123456.867 |
+------------+
3 rows in set (0.00 sec)

mysql> select  * from tt where  num=123456.867;
+------------+
| num        |
+------------+
| 123456.867 |
+------------+
1 row in set (0.00 sec)

mysql> insert into tt(num)values(2.8);
Query OK, 1 row affected (0.04 sec)

mysql> select * from   tt;
+------------+
| num        |
+------------+
| 123456.797 |
| 123456.797 |
| 123456.867 |
|      2.800 |
+------------+
4 rows in set (0.00 sec)

mysql> select  * from tt where  num=2.8;
+-------+
| num   |
+-------+
| 2.800 |
+-------+
1 row in set (0.00 sec)

mysql> insert into tt(num)values(2.888888);
Query OK, 1 row affected (0.00 sec)

mysql> select  * from  tt;
+------------+
| num        |
+------------+
| 123456.797 |
| 123456.797 |
| 123456.867 |
|      2.800 |
|      2.889 |
+------------+
5 rows in set (0.00 sec)
注：小數位數超了，自動取近似值。

--------------------------------------------------------------------------------------

2.double類型

mysql> create table tt(
    -> num  double(9,3)
    -> );
Query OK, 0 rows affected (0.02 sec)

mysql> insert into tt(num) values(234563.9);
Query OK, 1 row affected (0.00 sec)

mysql> select * from  tt;
+------------+
| num        |
+------------+
| 234563.900 |
+------------+
1 row in set (0.00 sec)

mysql> insert into tt(num) values(2345623.2);
ERROR 1264 (22003): Out of range value for column 'num' at row 1
mysql> insert into tt(num) values(234563.2);
Query OK, 1 row affected (0.00 sec)

mysql> select  * from  tt;
+------------+
| num        |
+------------+
| 234563.900 |
| 234563.200 |
+------------+
2 rows in set (0.00 sec)

mysql> insert into tt(num) values(2.8);
Query OK, 1 row affected (0.00 sec)

mysql> select  * from tt;
+------------+
| num        |
+------------+
| 234563.900 |
| 234563.200 |
|      2.800 |
+------------+
3 rows in set (0.00 sec)

FLOAT(M,D)或REAL(M,D)或DOUBLE PRECISION(M,D)。這里，“(M,D)”表示該值一共顯示M位整數，其中D位位于小數點后面。
例如，定義為FLOAT(7,4)的一個列可以顯示為-999.9999。MySQL保存值時進行四舍五入，因此如果在FLOAT(7,4)列內插入999.00009，近似結果是999.0001。