獲得此基礎上爲所有表

Question

的共同字段中的值從不同的表中的數據我有5張桌子......，

活動，用戶，管理員，個人，僱主

活動，管理，個人，僱主屬於用戶表所示

Activity   Admin     Individual    Employer 
+-----------+ +------------------+ +------------------+ +------------------+ 
id | user_id  id | user_id | name id | user_id | name  id | user_id | name 
+-----------+ +------------------+ +------------------+ +------------------+ 

現在，我射擊的Activity表的查詢，

如果user_id值在Admin表，然後我需要從Admin表， 獲取名稱。如果user_id值在Employer表，然後我需要從Employer表中獲取的名稱， 同樣的，如果user_id值在Individual表，然後我需要得到名稱來自Individual表，

Answer 1

使用LEFT JOIN與COALESCE。事情是這樣的：

SELECT 
    a.id, 
    COALESCE(d.name, i.name, e.name) AS userName 
FROM Activity  AS a 
LEFT JOIN admin  AS d ON a.user_id = d.user_id 
LEFT JOIN Individual AS i ON a.user_id = i.user_id 
LEFT JOIN Employer AS e ON a.user_id = e.user_id; 

Answer 2

嘗試這個概念：

SELECT 
    A.ID AS ID 
    A.User_id AS UserID 
    A.Name AS Name 
    'Admin' AS TableName 
FROM Admin A INNER JOIN Activity B 
ON A.User_Id=B.User_Id 

UNION ALL 

SELECT 
    A.ID AS ID 
    A.User_id AS UserID 
    A.Name AS Name 
    'Individual' AS TableName 
FROM Individual A INNER JOIN Activity B 
ON A.User_Id=B.User_Id 

UNION ALL 

SELECT 
    A.ID AS ID 
    A.User_id AS UserID 
    A.Name AS Name 
    'Employer' AS TableName 
FROM Employer A INNER JOIN Activity B 
ON A.User_Id=B.User_Id