Question

我有一个名为basicInfo的表，其中包含有关用户的基本信息（例如其性别）。

然后我创建了两个名为userMan和userWoman的表。

使用＆＃39;性别＆＃39;在basicInfo中，我想从userMan或userWoman中检索信息。

例如

SELECT
   IF(user.gender='male', (man.height, man.location), (woman.height, woman.location) 
FROM 
   basicInfo AS user 
    LEFT JOIN userMan AS man ON user.id=man.id
    LEFT JOIN userWoman AS woman ON user.id=woman.id;

或者

SELECT
    CASE WHEN user.gender='male'
     THEN man.height, man.location
     ELSE woman.height, woman.location
    END
FROM 
   basicInfo AS user 
    LEFT JOIN userMan AS man ON user.id=man.id
    LEFT JOIN userWoman AS woman ON user.id=woman.id;

我有大约17个值放在IF或CASE语句中。我必须使用＆＃34; user.gender =＆＃39;男性＆＃39;＆＃34;每17个值。

我考虑过使用UNION ALL。但是，我想从userman和userwoman检索的列是不同的。

有没有比使用＆＃34; user.gender =＆＃39;男性＆＃39;＆＃34;更好的方法。每列？

Answer 1

在这种情况下，使用union：

会更方便

SELECT
    user.gender,
    man.height, 
    man.location
FROM 
   basicInfo AS user 
    LEFT JOIN userMan AS man ON user.id=man.id
WHERE user.gender='male'
UNION
SELECT
    user.gender,
    woman.height, 
    woman.location
FROM 
   basicInfo AS user 
    LEFT JOIN userWoman AS woman ON user.id=woman.id;
WHERE user.gender='female'

Answer 2

除了似乎是最佳选择的UNION ALL之外，您还可以使用：

SELECT
   user.gender, 
   COALESCE(man.height, woman.height) AS height,
   COALESCE(man.location, woman.location) AS location
FROM 
   basicInfo AS user 
      LEFT JOIN userMan AS man 
         ON  user.id = man.id
         AND user.gender = 'male'
      LEFT JOIN userWoman AS woman 
         ON  user.id = woman.id
         AND user.gender = 'female'
WHERE                                     -- this can probably be removed
   user.gender IN ('male', 'female') ;     -- as well

如何在IF或CASE WHEN语句中选择多个值

2 个答案: