Mysql 使用if语句从4个不同的表中选择记录

这里userid将包含uid和前缀，对于staff，前缀是s

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对于用户，它是p，我想获取发布查询的用户详细信息。

您可以尝试以下操作：

 SELECT sup.qid,sup.title ,SUBSTRING(sup.userid, 1, 1) as t ,
 CASE WHEN SUBSTRING(sup.userid, 1, 1) = "s" 
 THEN  select name from staff st where st.id= SUBSTRING(sup.userid, 2, 1) 
 ELSE 
     when SUBSTRING(sup.userid, 1, 1) = "p" 
           THEN  select name from users p where p.id= SUBSTRING(sup.userid, 2, 1)  
   END AS user name 
 FROM `support_queries` sup 

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我想把这个逻辑放在on子句中：

SELECT sup.qid,
       sup.title,
       SUBSTRING(sup.userid, 1, 1) AS t,
       CASE SUBSTRING(sup.userid, 1, 1)
         WHEN 's' THEN staff.name
         WHEN 'p' THEN users.name
         ELSE NULL
       END AS user_name
FROM   support_queries sub
LEFT OUTER JOIN users
             ON SUBSTRING(sup.userid, 2, LENGTH(sup.userid) - 1) = users.id
LEFT OUTER JOIN staff
             ON SUBSTRING(sup.userid, 2, LENGTH(sup.userid) - 1) = staff.id

或者更简单地说：

SELECT sq.qid, sq.title, LEFT(sup.userid, 1) AS t,
       COALESCE(s.name, u.name) as user_name
FROM support_queries sq LEFT JOIN
     users u
     ON SUBSTRING(sq.userid, 2) = u.id AND
        LEFT(sq.userid, 1) = 'p' LEFT JOIN
     staff s
     ON SUBSTRING(sq.userid, 2) = s.id AND
        LEFT(sq.userid, 1) = 's';

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请提供示例数据和示例数据的预期结果。您使用的是MySQL还是MS SQL Server？不要标记未涉及的产品。@jarlh语法看起来更像mysql，让我们删除othera case表达式不能用作流控制。它只能返回一个值。此外，子查询只能返回单个值（除非用于in、exists或作为派生表）。

SELECT sq.qid, sq.title, LEFT(sup.userid, 1) AS t,
       COALESCE(s.name, u.name) as user_name
FROM support_queries sq LEFT JOIN
     users u
     ON sq.userid = CONCAT('p', u.id) LEFT JOIN
     staff s
     ON sq.userid = CONCAT('s', s.id);