SQL多表的设计查询,5种方法

都是根据brand_id是否等于brand.id来判断的

1.笛卡尔乘积

SELECT * FROM `products` , `brand`;
-- 获取到的是笛卡尔乘积进行筛选
SELECT * FROM `products` , `brand` WHERE products.brand_id = brand.id;

2.左连接

-- 2.1查询所有的手机(包括没有品牌信息的手机)以及对应的品牌 null
SELECT * FROM `products` LEFT OUTER JOIN `brand` ON products.brand_id = brand.id;
-- 2.2查询没有品牌信息的手机
SELECT * FROM `products` LEFT JOIN `brand` ON products.brand_id = brand.id WHERE brand.id IS NULL;

3.右连接

-- 2.1查询所有的品牌(没有对应的手机数据，品牌也显示)以及对应的手机数据
SELECT * FROM `products` RIGHT OUTER JOIN `brand` ON products.brand_id = brand.id;
-- 2.2查询没有对应手机的品牌信息
SELECT * FROM `products` RIGHT JOIN `brand` ON products.brand_id = brand.id WHERE products.brand_id IS NULL;

4.内连接

SELECT * FROM `products` JOIN `brand` ON products.brand_id = brand.id;

5.全连接

(SELECT * FROM `products` LEFT OUTER JOIN `brand` ON products.brand_id = brand.id)
UNION
(SELECT * FROM `products` RIGHT OUTER JOIN `brand` ON products.brand_id = brand.id)