Mysql|Natural Joins | Cross Joins | 自然连接 | 交叉连接

Natural join

SELECT 
	o.order_id,
    c.first_name
FROM orders o
NATURAL JOIN customers c 

数据库会自动将不同的两个列表中  相同列相同值的两列自动连接，但不鼓励使用，因为不能人为的选择

Cross Joins

SELECT 
	 c.first_name as customer,
     p.name as product
FROM customers c
CROSS JOIN products p
order by c.first_name

顾客表里的每条记录都会和产品表里的每条记录结合  ，类似于笛卡尔积

另外一个隐式写法

SELECT 
	c.first_name AS customer,
    p.name AS product
FROM customers c ,products p
-- CROSS JOIN products p
ORDER BY c.first_name

练习

做一个发货人和产品的交叉连接。先使用隐性连接 后使用显性连接

隐式

SELECT 
	 s.name as shipper,
     p.name as product
FROM shippers s, products p
ORDER BY s.name;

显示

SELECT 
	 s.name as shipper,
     p.name as product
FROM shippers s
CROSS JOIN products p
ORDER BY s.name