SELECT cust_id
FROM orders
WHERE order_num IN(
SELECT order_num
FROM orderitems
WHERE prod_id=‘TNT2‘
)
)
2、计算字段中
例2：查询每个客户的订单总数
SELECT cust_id,cust_name,cust_contact,
(SELECT COUNT(*)
FROM orders
WHERE orders.cust_id=customers.cust_id)
FROM customers ORDER BY cust_name
MySQL：子查询
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