MYSQL從2個表中進行選擇，這些表的ID僅在其中一個表中

Question

這是我的問題：我有3個表：

USERS

USERS_INFO

COMPANIES_INFO

USERS表具有字段ID 。表USERS_INFO和COMPANIES_INFO具有由ID為的外鍵鏈接的ID_USER字段。

問題是，如何選擇僅在其中一個中存在的行？

一個例子：

USERS
+----+
| id |
+----+
| 1  | 
+----+
| 2  | 
+----+

USERS_INFO
+---------+---------+
| id_user | name    |
+---------+---------+
| 1       | Jhonny  |
+---------+---------+


COMPANIES_INFO
+---------+---------+
| id_user | company |
+---------+---------+
| 2       | Apple   |
+---------+---------+

我想要的是這樣的：

SELECT * FROM users_info, companies_info WHERE id_user=2

並得到這個：

id_user = 2
company = Apple

相反，如果我做到了

SELECT * FROM users_info, companies_info WHERE id_user=1

我會得到：

id_user =1
name = Jhonny

例如，我想通過檢查兩個表USERS_INFO和COMPANIES_INFO來選擇用戶2，因為我們不知道哪個表包含該表……有什么幫助嗎？

Answer 1

嘗試這個：

     SELECT U.id
     FROM users U
     LEFT JOIN users_info UI ON U.id = CI.id_user
     LEFT JOIN companies_info CI ON U.id = CI.id_user
     WHERE UI.id IS NOT NULL AND CI.id IS NOT NULL

Answer 2

您可以嘗試以下方法：

SET @user = 1;
SELECT id_user, name FROM USERS_INFO WHERE id_user = @user
UNION
SELECT id_user, company as name FROM COMPANIES_INFO WHERE id_user = @user;