MySQL 如何从另一个字段推导出字段的值

为此，您可以使用用户定义变量的概念。让我们首先创建一个表 –

mysql> create table DemoTable1868
    (
    Value int
    );
Query OK, 0 rows affected (0.00 sec)

使用插入命令在表中插入一些记录 –

mysql> insert into DemoTable1868 values(10);
Query OK, 1 row affected (0.00 sec)
mysql> insert into DemoTable1868 values(20);
Query OK, 1 row affected (0.00 sec)
mysql> insert into DemoTable1868 values(30);
Query OK, 1 row affected (0.00 sec)
mysql> insert into DemoTable1868 values(40);
Query OK, 1 row affected (0.00 sec)

使用select语句显示表中的所有记录 –

mysql> select * from DemoTable1868;

这将产生以下输出 –

+-------+
| Value |
+-------+
| 10 |
| 20 |
| 30 |
| 40 |
+-------+
4 rows in set (0.00 sec)

以下是在MySQL中从另一个字段派生字段值的查询 –

mysql> select  Value,(@iterator:=Value+@iterator) as SecondColumn from DemoTable1868,( select @iterator:=0) tbl;

这将产生以下输出 –

+-------+--------------+
| Value | SecondColumn |
+-------+--------------+
| 10 | 10 |
| 20 | 30 |
| 30 | 60 |
| 40 | 100 |
+-------+--------------+
4 rows in set (0.00 sec)