11子查询

子查询

嵌套在查询中的查询

1.利用子查询进行过滤

SELECT cust_name, cust_contact
From Customers
Where cust_id IN (SELECT cust_id
                 FROM Orders
                 Where order_num IN (SELECT order_num
                                    From order_Items
                                    Where prod_id = 'RGAN01'));

子查询的SELECT只能查询单列

2.作为计算字段使用子查询

将子查询作为一列需要显示的内容（即计算字段），来与另一张表联动

SELECT cust_name,cust_status,
		(SELECT COUNT(*) 
        FROM Orders 
        Where Orders.cust_id = Customers.cust_id) AS orders
From Customers
Order by cust_name;

发布于 2022-10-05