MySQL 如何得到两个列的差值

首先，让我们创建一个带有要在新列中计算差值的列的表 –

mysql> create table DemoTable
(
   Id int NOT NULL AUTO_INCREMENT PRIMARY KEY,
   LowValue int,
   HighValue int
);
Query OK, 0 rows affected (0.62 sec)

使用insert命令向表中插入一些记录 –

mysql> insert into DemoTable(LowValue,HighValue) values(100,200);
Query OK, 1 row affected (0.19 sec)
mysql> insert into DemoTable(LowValue,HighValue) values(300,700);
Query OK, 1 row affected (0.12 sec)
mysql> insert into DemoTable(LowValue,HighValue) values(1000,2000);
Query OK, 1 row affected (0.13 sec)

以下是使用select语句显示表中所有记录的查询 –

mysql> select *from DemoTable;

这将产生以下输出 –

+----+----------+-----------+
| Id | LowValue | HighValue |
+----+----------+-----------+
| 1  | 100      | 200       |
| 2  | 300      | 700       |
| 3  | 1000     | 2000      |
+----+----------+-----------+
3 rows in set (0.00 sec)

以下是获取两个列之间差异的查询 –

mysql> select tbl.Id,tbl.LowValue,tbl.HighValue, 
   HighValue-LowValue AS DifferenceBetweenTwoColumns from DemoTable tbl;

这将产生以下输出 –

+----+----------+-----------+-----------------------------+
| Id | LowValue | HighValue | DifferenceBetweenTwoColumns |
+----+----------+-----------+-----------------------------+
| 1  | 100      | 200       | 100                         |
| 2  | 300      | 700       | 400                         |
| 3  | 1000     | 2000      | 1000                        |
+----+----------+-----------+-----------------------------+
3 rows in set (0.00 sec)

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