将第二个 MySQL 表中的数据添加到第一个表的结果中

Question

我有 2 张桌子：

+-----------+
|   users   |
+-----------+
|   John    |
|   Peter   |
|   Alex    |
+-----------+

和

+-----------+
|   banned  |
+-----------+
|   John    |
|   Peter   |
+-----------+

是否可以SELECT * FROM users 并将结果与​​第二个表结合起来，以确定用户是否被禁止？

Answer 1

您可以使用LEFT JOIN：

-- with CASE WHEN
SELECT users.*, CASE WHEN banned.username IS NULL THEN 0 ELSE 1 END AS isBanned
FROM users LEFT JOIN banned ON users.username = banned.username

-- without CASE WHEN (thanks to @forpas!)
SELECT users.*, banned.username IS NOT NULL AS isBanned
FROM users LEFT JOIN banned ON users.username = banned.username

如果用户与banned 表不匹配，则用户名（以及其他列也是）为NULL。您可以使用CASE WHEN 来检查用户名是否为NULL。

您也可以使用EXISTS 代替LEFT JOIN 来获取这些附加信息：

SELECT *, EXISTS(SELECT 1 FROM banned WHERE users.username = banned.username) AS isBanned
FROM users

还有第三个选项使用IF：

SELECT users.*, IF(banned.username IS NULL, 0, 1) AS isBanned
FROM users LEFT JOIN banned ON users.username = banned.username

demo on dbfiddle.uk