mysql(3)

1.子查询

 select cust_id from orders
 where order_num in (select order_num from orderitems 
                                where prod_id=‘TNT2‘)

SELECT cust_id,cust_name,(select count(*) from orders where orders.cust_id=
customers.cust_id)  from customers;

等效于select q.cust_id,q.cust_name ,count(order_num) from  orders p right join  customers q  
on p.cust_id=q.cust_id group by q.cust_id

2.等值连接（内部连接）

mysql(3)

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