Mysql Select case case and field comparison

Question

我從表中選擇字段比較值，例如：

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

並且現在想要實際選擇賦予差值字段的值，例如，

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

等，但我無法如何插入選擇。我已經在更新這樣做是成功的，但選擇逃避我

Answer 1

我覺得一個Sub-Select查詢將工作做好在這裏。

目前你的說法是這樣的：

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

相反，你可以在這個子選擇這樣的：

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

現在你可以做你的case語句在最外層的SELECT語句：

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> 
) 

Answer 2

一種方法是使用子查詢：

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 
)