Question

我从具有字段比较值的表中进行选择，例如

Select FieldA,FieldB,(FieldC-FIeldD) as Difference

现在想要实际选择赋予差异字段的值，例如

When Difference=0 then 'North'
When Different>0 and Difference<=90 then 'NorthEast'
When Difference=90 then 'South'

等但是我无法将其插入到Select中。我已经在更新中成功完成了这项工作，但Select退出了我

Answer 1

一种方法是使用子查询：

SELECT FieldA, FieldB, 
CASE 
   WHEN Difference = 0 THEN 'North'
   WHEN Difference > 0 AND <= 90 THEN 'NorthEast'
   WHEN Difference = 90 THEN'South'
END AS Foo
FROM (
     Select FieldA, FieldB, (FieldC-FIeldD) AS Difference
)

Answer 2

我认为Sub-Select查询在这里运作良好。

目前您的陈述如下：

Select FieldA,FieldB,(FieldC-FIeldD) as Difference from <TABLE>

相反，你可以在这样的子选择中使用它：

select * from 
(
Select FieldA,FieldB,(FieldC-FIeldD) as Difference from <TABLE>
) sub_table

现在，您可以在最外面的select语句中执行case语句：

select 
CASE
    When Difference=0 then 'North'
    When Difference>0 and Difference<=90 then 'NorthEast'
    When Difference=90 then 'South'
END as answer
from 
(
Select FieldA,FieldB,(FieldC-FIeldD) as Difference from <TABLE>
)

Mysql选择case语句和字段比较

2 个答案: