MySQL必知必会：第十四章 使用子查询

1.利用子查询进行过滤

子查询总是从内向外处理

#列出订购TNT2的全部客户信息
SELECT * FROM customers WHERE cust_id IN (
	SELECT cust_id FROM orders WHERE order_num IN (
		SELECT order_num FROM orderitems WHERE prod_id = 'TNT2'))

2.作为计算字段使用子查询

#列出每个客户订单数量
SELECT cust_name,
			(SELECT COUNT(*) FROM orders 
			WHERE orders.cust_id=customers.cust_id) AS orders
FROM customers
ORDER BY cust_name

#比上面的好理解一点
SELECT a.cust_id,a.cust_name,b.orders 
FROM customers a 
LEFT JOIN
				(SELECT cust_id,COUNT(*) AS orders
				FROM orders 
				GROUP BY cust_id) b
ON b.cust_id=a.cust_id

相关子查询：涉及外部的子查询

任何时候只要列名可能有多义性，就要使用这种语法

orders.cust_id=customers.cust_id 列名相同时，前面要加上表名