是否有可能有一個空的MySQL數值字段？

Question

我想在MySQL數字字段上有一個空字段。如果我將該字段定義爲允許NULL值，則默認爲0.00。有時候，對於這一行項目，我更喜歡沒有價值我大概可以創建一個不同的表來跟蹤這些項目，但是在這一點上我更喜歡一個表格解決方案。

Answer 1

因爲你是用DEFAULT做到這一點的。不要做：

create table t1 
( id int auto_increment primary key, 
    thing varchar(100) not null, 
    anInt NUMERIC(5,2) NULL DEFAULT 0 
); 

insert t1(thing) values ('fish'); 
select * from t1; 
+----+-------+-------+ 
| id | thing | anInt | 
+----+-------+-------+ 
| 1 | fish | 0.00 | 
+----+-------+-------+ 

Answer 2

對我的作品在MySQL 5.7.12：

mysql> create table X (a double null); 
Query OK, 0 rows affected (0.01 sec) 

mysql> desc X; 
+-------+--------+------+-----+---------+-------+ 
| Field | Type | Null | Key | Default | Extra | 
+-------+--------+------+-----+---------+-------+ 
| a  | double | YES |  | NULL |  | 
+-------+--------+------+-----+---------+-------+ 
1 row in set (0.00 sec) 

mysql> alter table X add column b int; 
Query OK, 0 rows affected (0.01 sec) 
Records: 0 Duplicates: 0 Warnings: 0 

mysql> desc X; 
+-------+---------+------+-----+---------+-------+ 
| Field | Type | Null | Key | Default | Extra | 
+-------+---------+------+-----+---------+-------+ 
| a  | double | YES |  | NULL |  | 
| b  | int(11) | YES |  | NULL |  | 
+-------+---------+------+-----+---------+-------+ 
2 rows in set (0.00 sec) 

mysql> insert into X (a) values (1.3); 
Query OK, 1 row affected (0.00 sec) 

mysql> insert into X (b) values (1); 
Query OK, 1 row affected (0.00 sec) 

mysql> select * from X; 
+------+------+ 
| a | b | 
+------+------+ 
| 1.3 | NULL | 
| NULL | 1 | 
+------+------+ 
2 rows in set (0.00 sec)