MySQL中多表联查JOIN的常见用法详解

-- 用户表 user
CREATE TABLE user(
  id INT PRIMARY KEY AUTO_INCREMENT,
  name VARCHAR(20),
  age INT
);

-- 订单表 order
CREATE TABLE `order`(
  id INT PRIMARY KEY AUTO_INCREMENT,
  user_id INT, -- 外键，关联user.id
  order_name VARCHAR(50)
);

SELECT 字段
FROM 表A
INNER JOIN 表B
ON 表A.关联字段 = 表B.关联字段;

SELECT u.name, o.order_name
FROM user u
INNER JOIN `order` o
ON u.id = o.user_id;

SELECT u.name, o.order_name
FROM user u,`order` o
WHERE u.id = o.user_id;

SELECT u.name, o.order_name
FROM user u
LEFT JOIN `order` o
ON u.id = o.user_id;

SELECT u.name
FROM user u
LEFT JOIN `order` o
ON u.id = o.user_id
WHERE o.id IS NULL;

SELECT u.name, o.order_name
FROM user u
RIGHT JOIN `order` o
ON u.id = o.user_id;

SELECT u.name, o.order_name FROM user u LEFT JOIN `order` o ON u.id=o.user_id
UNION
SELECT u.name, o.order_name FROM user u RIGHT JOIN `order` o ON u.id=o.user_id;

SELECT u.name,o.order_name,g.goods_name
FROM user u
LEFT JOIN `order` o ON u.id = o.user_id
LEFT JOIN goods g ON o.goods_id = g.id;

-- where中子查询
SELECT * FROM user WHERE id IN (SELECT user_id FROM `order`);

-- from子查询（派生表）
SELECT * FROM (SELECT * FROM user WHERE age>18) t1
LEFT JOIN `order` o ON t1.id = o.user_id;

-- ❌错误：left join后where过滤order表，会把null数据过滤掉，退化成内连接
SELECT u.name,o.order_name FROM user u
LEFT JOIN `order` o ON u.id=o.user_id
WHERE o.order_name='手机';

-- ✅正确，把条件放on
SELECT u.name,o.order_name FROM user u
LEFT JOIN `order` o ON u.id=o.user_id AND o.order_name='手机';