第十四章：使用子查询

@author: Tobin
@date: 2019/10/31 17:44:42

MySQL4.1引入了对子查询的支持。

# 嵌套
SELECT cust_name, cust_contact
FROM customers
WHERE cust_id IN (SELECT cust_id 
                                 FROM orders
                                 WHERE order_num IN (SELECT order_num
                                                                         FROM orderitems
                                                                         WHERE prod_id = 'TNT2'));

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