[轉]MySQL 數據類型（float）的注意事項

http://www.cnblogs.com/zhoujinyi/archive/2013/04/26/3043160.html

可能由於版本關系，我的mysql5.7插入數據超過范圍時會提示，1264 - Out of range value for column 'price' at row 1

摘要:
      今天左哥問起一個float浮點數類型的問題，這個類型用的不多，所以也不太了解，現在打算測試下。

知識點：
      float：浮點數，單精度，占4字節。

測試

root@localhost : test 05:49:32>create table fl(id int,fl float); Query OK, 0 rows affected (0.05 sec) root@localhost : test 05:49:40>insert into fl values(1,123456); Query OK, 1 row affected (0.00 sec) root@localhost : test 05:49:51>insert into fl values(2,123.456); Query OK, 1 row affected (0.00 sec) root@localhost : test 05:49:57>insert into fl values(3,1234.567);#7位了 Query OK, 1 row affected (0.00 sec) root@localhost : test 05:50:13>insert into fl values(4,1234567);#7位了 Query OK, 1 row affected (0.00 sec) root@localhost : test 05:50:26>select * from fl; +------+-------------+ | id | fl | +------+-------------+ | 1 | 123456 | | 2 | 123.456 | | 3 | 1234.57 | | 4 | 1.23457e+06 | +------+-------------+ 4 rows in set (0.00 sec)

從上面看出：默認的float類型都只能存6個數字（包括小數點前后的位數），整數超過6位就被科學計數表示（id=4），小數位超出則需要四舍五入。那指定float(m)的精度呢？

root@localhost : test 05:59:08>create table fl(id int,fl float(20)); Query OK, 0 rows affected (0.04 sec) root@localhost : test 05:59:28>show create table fl\G; *************************** 1. row *************************** Table: fl Create Table: CREATE TABLE `fl` ( `id` int(11) DEFAULT NULL, `fl` float DEFAULT NULL ) ENGINE=MyISAM DEFAULT CHARSET=utf8 1 row in set (0.00 sec) root@localhost : test 05:59:35>insert into fl values(1,123456); Query OK, 1 row affected (0.00 sec) root@localhost : test 05:59:44>insert into fl values(2,123.456); Query OK, 1 row affected (0.00 sec) root@localhost : test 05:59:46>insert into fl values(3,1234.567);#7位了 Query OK, 1 row affected (0.00 sec) root@localhost : test 05:59:50>insert into fl values(4,1234567);#7位了 Query OK, 1 row affected (0.00 sec) root@localhost : test 05:59:53>select * from fl; +------+-------------+ | id | fl | +------+-------------+ | 1 | 123456 | | 2 | 123.456 | | 3 | 1234.57 | | 4 | 1.23457e+06 | +------+-------------+ 4 rows in set (0.01 sec)

從上面看出：和默認的float一樣。所以：默認float和float(m)一樣 ，m<=24； 都是默認float類型，都只能存6個數字（包括小數點前后的位數），整數超過6位就被科學計數表示（id=4）；小數位超出則需要四舍五入。那指定他的標度float(m,d) 呢？

mysql> show create table fl\G; *************************** 1. row *************************** Table: fl Create Table: CREATE TABLE `fl` ( `id` int(11) DEFAULT NULL, `fl` float(7,4) DEFAULT NULL ) ENGINE=MyISAM DEFAULT CHARSET=utf8 1 row in set (0.02 sec) ERROR: No query specified mysql> insert into fl values(1,123456);#整數是3位，超出了 Query OK, 1 row affected, 1 warning (0.00 sec) mysql> insert into fl values(3,123.4567); Query OK, 1 row affected (0.00 sec) mysql> insert into fl values(2,123.45678);#小數位超出，四舍五入 Query OK, 1 row affected (0.00 sec) mysql> insert into fl values(4,12.456789);#小數位超出，四舍五入 Query OK, 1 row affected (0.00 sec) mysql> select * from fl; +------+----------+ | id | fl | +------+----------+ | 1 | 999.9999 | | 3 | 123.4567 | | 2 | 123.4568 | | 4 | 12.4568 | +------+----------+ 4 rows in set (0.00 sec)

從上面看出：float(m,d)：小數點后位數為d，即整數位數為（m-d），整數位超出則整數為(m-d)個999，小數點后位數為(d)個9999，不用科學計算了；若小數位超出，則需要四舍五入。要是float(m,d)遇到m和d一樣呢？

mysql> CREATE TABLE `fl` ( -> `id` int(11) DEFAULT NULL, -> `fl` float(4,4) DEFAULT NULL -> ) ENGINE=MyISAM DEFAULT CHARSET=utf8; Query OK, 0 rows affected (0.04 sec) mysql> insert into fl values(1,123456); Query OK, 1 row affected, 1 warning (0.00 sec) mysql> insert into fl values(2,123.45678); Query OK, 1 row affected, 1 warning (0.00 sec) mysql> insert into fl values(3,123.4567); Query OK, 1 row affected, 1 warning (0.00 sec) mysql> insert into fl values(4,12.456789); Query OK, 1 row affected, 1 warning (0.00 sec) mysql> select * from fl; +------+--------+ | id | fl | +------+--------+ | 1 | 0.9999 | | 2 | 0.9999 | | 3 | 0.9999 | | 4 | 0.9999 | +------+--------+ 4 rows in set (0.00 sec)

從上面看出：float(m,d)：若m和d一樣，則先把小數位數先滿足了，再分配整數位數。所以只有小數點位數，而整數位是0。

之前提過的m<=24，要是大於24呢？

mysql> CREATE TABLE `fl` ( -> `id` int(11) DEFAULT NULL, -> `fl` float(25) DEFAULT NULL -> ) ENGINE=MyISAM DEFAULT CHARSET=utf8; Query OK, 0 rows affected (0.01 sec) mysql> show create table fl\G; *************************** 1. row *************************** Table: fl Create Table: CREATE TABLE `fl` ( `id` int(11) DEFAULT NULL, `fl` double DEFAULT NULL ) ENGINE=MyISAM DEFAULT CHARSET=utf8 1 row in set (0.00 sec)

從上面看出：在沒有標度的float中，從float轉換成了double。而有標度的 float(m,d)當中，m>24呢？

mysql> CREATE TABLE `fl` ( -> `id` int(11) DEFAULT NULL, -> `fl` float(100,3) DEFAULT NULL -> ) ENGINE=MyISAM DEFAULT CHARSET=utf8; Query OK, 0 rows affected (0.01 sec) mysql> show create table fl\G; *************************** 1. row *************************** Table: fl Create Table: CREATE TABLE `fl` ( `id` int(11) DEFAULT NULL, `fl` float(100,3) DEFAULT NULL ) ENGINE=MyISAM DEFAULT CHARSET=utf8

從上面看出：從float還是float，沒有受到影響。不過設置這么大，在整數會變的不准；小數的位數還是m-d，位數不足用0填充（zerofill）。如下測試：

mysql> show create table fl\G; *************************** 1. row *************************** Table: fl Create Table: CREATE TABLE `fl` ( `id` int(11) DEFAULT NULL, `fl` float(100,3) DEFAULT NULL ) ENGINE=MyISAM DEFAULT CHARSET=utf8 1 row in set (0.00 sec) 
mysql> insert into fl values(5,777777777777777777777777777.5555);
Query OK, 1 row affected (0.00 sec)

mysql> select * from fl;
+------+---------------------------------+
| id   | fl                              |
+------+---------------------------------+
|    5 | 777777744225350500000000000.000 | #不准了
+------+---------------------------------+
1 row in set (0.00 sec)
 mysql> drop table fl; Query OK, 0 rows affected (0.00 sec) mysql> CREATE TABLE `fl` ( -> `id` int(11) DEFAULT NULL, -> `fl` float(25,23) DEFAULT NULL -> ) ENGINE=MyISAM DEFAULT CHARSET=utf8; Query OK, 0 rows affected (0.02 sec) mysql> show create table fl\G; *************************** 1. row *************************** Table: fl Create Table: CREATE TABLE `fl` ( `id` int(11) DEFAULT NULL, `fl` float(25,23) DEFAULT NULL ) ENGINE=MyISAM DEFAULT CHARSET=utf8 1 row in set (0.00 sec) mysql> insert into fl values(5,99.555555555555555); Query OK, 1 row affected (0.00 sec) mysql> select * from fl; +------+----------------------------+ | id | fl | +------+----------------------------+ | 5 | 99.55555725097656000000000 | #小數位被0填充 +------+----------------------------+ 1 row in set (0.00 sec)

注意：當float(m,d) 設置了標度之后，小數位一定會存在。當整數位沒有被超過，需要用0填充；整數位被超過了，需要用9填充，小數位數不能大於30否則報錯：

mysql> CREATE TABLE `fl` ( -> `id` int(11) DEFAULT NULL, -> `fl` float(5,3) DEFAULT NULL -> ) ENGINE=MyISAM DEFAULT CHARSET=utf8 -> ; Query OK, 0 rows affected (0.03 sec) mysql> insert into fl values(1,123); Query OK, 1 row affected, 1 warning (0.00 sec) mysql> select * from fl; +------+--------+ | id | fl | +------+--------+ | 1 | 99.999 | +------+--------+ 1 row in set (0.00 sec) mysql> insert into fl values(2,13); Query OK, 1 row affected (0.00 sec) mysql> insert into fl values(3,3); Query OK, 1 row affected (0.00 sec) mysql> select * from fl; +------+--------+ | id | fl | +------+--------+ | 1 | 99.999 |  #9填充 | 2 | 13.000 | #0填充 | 3 | 3.000 | +------+--------+ 3 rows in set (0.00 sec)

mysql> CREATE TABLE `fl` ( -> `id` int(11) DEFAULT NULL, -> `fl` float(102,100) DEFAULT NULL -> ) ENGINE=MyISAM DEFAULT CHARSET=utf8 -> ) ENGINE=MyISAM DEFAULT CHARSET=utf8; ERROR 1425 (42000): Too big scale 100 specified for column 'fl'. Maximum is 30. #標度不能大於30，等於30可以建立 mysql> CREATE TABLE `fl` ( -> `id` int(11) DEFAULT NULL, -> `fl` float(52,50) DEFAULT NULL -> ) ENGINE=MyISAM DEFAULT CHARSET=utf8; ERROR 1425 (42000): Too big scale 50 specified for column 'fl'. Maximum is 30.

總結：float存在這么多潛在因素，用的時候需要注意，能避免就避免用。