如何连接两个表，其中一个表中的一列是指其他表中的3列？

Question

我有两个表用户和User_activity和他们的结构，

用户

id 
name 
email 
etc.. 

user_activity

id, 
initiator_id, 
source_id, 
target_id, 
activity_code, 
activity 

在这里，我要选择的所有活动列表为一个用户（我可以引用他们source_id）的所有名称的发起人，来源，目标..

如何获得？

Answer 1

您只需加入三次，使用别名按照每个F您引用用户的三倍......

SELECT 
    user_activity.*, 
    Initiator.Name AS InitiatorName, 
    Source.Name  AS SourceName, 
    Target.Name  AS TargetName 
FROM 
    user_activity 
INNER JOIN 
    user    AS Initiator 
    ON Initiator.ID = user_activity.initiator_id 
INNER JOIN 
    user    AS Source 
    ON Source.ID = user_activity.source_id 
INNER JOIN 
    user    AS Target 
    ON Target.ID = user_activity.target_id 

Answer 2

SELECT sourceUser.name, targetUser.name, initiatorUser.name FROM user_activity 
INNER JOIN User as sourceUser on sourceUser.id = user_activity.source_id 
INNER JOIN User as targetUser on targetUser.id = user_activity.target_id 
INNER JOIN User as initiatorUser on initiatorUser.id = user_activity.initiator_id 
WHERE source_id = MyUserIdQueryValue 

Answer 3

SELECT DT1.*, U.name 
    FROM (
     SELECT source_id, activity_code, activity 
      FROM user_activity 
     UNION 
     SELECT target_id AS source_id, activity_code, activity 
      FROM user_activity 
     UNION 
     SELECT inititator_id AS source_id, activity_code, activity 
      FROM user_activity 
     ) DT1 
     JOIN User U ON DT1.source_id = U.id;