基於字段值的mysql更新行

Question

如何根據以前的值更新mysql中的字段？

讓我們說count等於3

UPDATE MY_TABLE SET count = count-1 

請問這項工作？所以計數的新值將是4

Answer 1

您是否意指行或表？如果你的意思是插入或更新一行，你可以使用triggers。

Answer 2

我試了一下，它的工作，所以也許你需要向我們展示CREATE語句的表。測試結果：

mysql> create table aint (count int not null default 0); 
Query OK, 0 rows affected (0.25 sec) 

mysql> insert into aint VALUES(1); 
Query OK, 1 row affected (0.06 sec) 

mysql> select * from aint; 
+-------+ 
| count | 
+-------+ 
|  1 | 
+-------+ 
1 row in set (0.00 sec) 

mysql> update aint set count = count-1; 
Query OK, 1 row affected (0.06 sec) 
Rows matched: 1 Changed: 1 Warnings: 0 

mysql> select * from aint; 
+-------+ 
| count | 
+-------+ 
|  0 | 
+-------+ 
1 row in set (0.00 sec) 

mysql> update aint set count = count-1; 
Query OK, 1 row affected (0.00 sec) 
Rows matched: 1 Changed: 1 Warnings: 0 

mysql> select * from aint; 
+-------+ 
| count | 
+-------+ 
| -1 | 
+-------+ 
1 row in set (0.00 sec)