数据库学习——子查询与联结

子查询

SELECT product.id,product.`name` FROM product WHERE  product.`id` IN (SELECT product.`id` FROM product WHERE product.`id` IN (1,3,5,7,9))

将字句的结果作为父句的查询条件

联结

SELECT p1.`id`,p2.`id`,p1.`name`,p2.`name` FROM product AS p1 INNER JOIN product AS p2 ON p1.`id` = p2.`id`

使用联结查询两个表并使用ON关键字来提供筛选条件

联结条件

SELECT Customers.cust_id,COUNT(Orders.order_num) AS num_ordFROM Customers LEFT OUTER JOIN OrdersON Customers.cust_id = Orders.cust_idGROUP BY Customers.cust_id;

筛选出所有顾客的订单数，包括没有下过单的顾客