MySQL使用子查询

子查询：即嵌套在其他查询中的查询

利用子查询进行过滤

把一条查询语句的结果用于另一条查询语句的WHERE子句

SELECT cust_id FROM orders where order_num IN (SELECT order_num FROM orderitems WHERE prod_id = 'TNT2');

作为计算字段使用子查询

把查询语句的结果作为另一条查询语句的计算字段返回。

SELECT cust_name, cust_state, (SELECT COUNT(*) FROM orders WHERE orders.cust_id = customers.cust_id) AS orders FROM customers ORDER BY orders;