第14章：使用子查询。

子查询是镶嵌在其他查询里面，相当其他的select查询的条件来。

P91 

select order_num from where prod_id=‘tnt2‘;   #检索条件为prod_id=tnt2的order_num#

select cust_id from orders where order_num in (20005,20007);    #检索满足条件 order_num在范围（20005，20007）的cust_id#

select cust_id from orders where order_num in (select order_num from orderitems where prod_id =‘tnt2‘ );  #检索 满足条件：///在表orderitems中满足prod_id=tnt2 的order_num，orderitems.order_num =orders.order_num  /// 这部分的order_num在orders中对应的cust_id # ##实际上，就是上面的两个语句组合而成的##

 P92

select cust_name,cust_contact from customers where cust_id in (10001,10004); #检索表customers满足客户Id在范围（10001，10004）内，对应的cust_name,cust_contact #

select cust_name,cust_contact from customers where cust_id in (select cust_id from orders where cust_num in (select cust_num from orderitems where prod_id =‘tnt2‘) ); #镶嵌了两个子查询，从最里面的括号的条件开始运行#