MySQL必知必会学习笔记（5）—— 子查询

1 利用子查询进行过滤

列出订购物品TNT2的所有客户

1. 检索包含物品TNT2的所有订单的编号；
2. 检索具有前一步骤列出的订单编号的所有客户的ID;
3. 检索前一步骤返回的所有客户ID的客户信息；

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select order_num
from orderitems
where prod_id='TNT2';

select cust_id
from orders
where order_num IN (20005,20007);

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

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SELECT cust_name,cust_contact
from customers
where cust_id IN (10001,10004);

2 作为计算字段使用子查询

问题：需要显示 customers 表中每个客户的订单总数。（订单与相应的客户ID存储在orders表中）

1. 从 customers 表中检索客户列表；
2. 对于检索出的每个客户，统计其在orders 表中的订单数目；

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SELECT cust_name,cust_state,
(SELECT COUNT(*) from orders WHERE
orders.cust_id=customers.cust_id) as orders
from customers
order by cust_name;