sql内连接中的原因条件

Question

她考虑这个城市查询中的条件的原因是什么？

SELECT salesman.name AS "Salesman",
customer.cust_name, customer.city 
FROM salesman,customer 
WHERE salesman.city=customer.city;

查询的目的是寻找同城的客户和卖家。可以在表中的FK字段上写查询吗？

SELECT salesman.name AS "Salesman",
customer.cust_name, customer.city 
FROM salesman,customer 
WHERE salesman.city=customer.city;

Answer 1

可能吗？是的（至少我是这么认为）。

示例数据（类似 Oracle，但忽略它）

SQL> with
  2  salesman (name, city) as
  3    (select 'Little', 'Berlin' from dual union all  --> Scott lives here
  4     select 'Foot'  , 'London' from dual union all  --> Mike lives here
  5     select 'Big'   , 'Zagreb' from dual
  6    ),
  7  customer (name, city) as
  8    (select 'Scott' , 'Berlin' from dual union all  --> Little lives here
  9     select 'Tiger' , 'Bern'   from dual union all
 10     select 'Mike'  , 'London' from dual            --> Foot lives here
 11    )

查询返回什么？

 12  select s.name salesman,
 13         c.name customer,
 14         s.city they_live_in
 15  from salesman s join customer c on c.city = s.city;

SALESMAN   CUSTOMER   THEY_LIVE_IN
---------- ---------- ---------------
Little     Scott      Berlin
Foot       Mike       London

SQL>

对我来说，看起来还不错。