如何在MySQL中实现当字段值>0时减1？

您可以在UPDATE命令中使用CASE语句来实现此操作。让我们先创建一个表−

mysql> create table DemoTable
(
    Id int NOT NULL AUTO_INCREMENT PRIMARY KEY,
    Value int
);
Query OK，0 rows affected (1.44 sec)

使用INSERT命令将记录插入表中−

mysql> insert into DemoTable(Value) values(100);
Query OK，1 row affected (0.47 sec)
mysql> insert into DemoTable(Value) values(0);
Query OK，1 row affected (4.16 sec)
mysql> insert into DemoTable(Value) values(104);
Query OK，1 row affected (0.16 sec)
mysql> insert into DemoTable(Value) values(0);
Query OK，1 row affected (0.15 sec)
mysql> insert into DemoTable(Value) values(5);
Query OK，1 row affected (0.20 sec)
mysql> insert into DemoTable(Value) values(10);
Query OK，1 row affected (0.24 sec)
mysql> insert into DemoTable(Value) values(0);
Query OK，1 row affected (0.18 sec)

使用SELECT命令从表中显示记录−

mysql> select *from DemoTable;

将产生以下输出−

+----+-------+
| Id | Value |
+----+-------+
| 1  | 100   |
| 2  | 0     |
| 3  | 104   |
| 4  | 0     |
| 5  | 5     |
| 6  | 10    |
| 7  | 0     |
+----+-------+
7 rows in set (0.00 sec)

现在使用设置条件，如果字段值大于0，则减去1；如果字段值≤0，则不变−

mysql> update DemoTable set Value=CASE WHEN Value > 0 THEN Value-1 ELSE 0 END;
Query OK，4 rows affected (0.25 sec)
Rows matched: 7 Changed: 4 Warnings: 0

让我们再次检查表中的记录−

mysql> select *from DemoTable;

将产生以下输出−

+----+-------+
| Id | Value |
+----+-------+
| 1  | 99    |
| 2  | 0     |
| 3  | 103   |
| 4  | 0     |
| 5  | 4     |
| 6  | 9     |
| 7  | 0     |
+----+-------+
7 rows in set (0.00 sec)

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