2、相关子查询.sql

本代码简单明了

--相关子查询
功能：销售数量低于该书平均销售数量的销售信息.






USE pubs
SELECT *
FROM sales s1
WHERE s1.qty <
   (SELECT AVG(s2.qty)
   FROM sales s2
   WHERE s2.title_id = s1.title_id)



--牛刀小试

功能: 查找已由多个出版商出版的书的类型


select * from titles





















USE pubs
SELECT DISTINCT t1.type
FROM titles t1
WHERE t1.type IN
   (SELECT t2.type
   FROM titles t2
   WHERE t1.pub_id <> t2.pub_id)



--自联接形式

USE pubs
SELECT DISTINCT t1.type
FROM titles t1 INNER JOIN titles t2 ON t1.type = t2.type 
   AND t1.pub_id <> t2.pub_id





--?联接和子查询的区别？


联接可以显示多个表的列，而子查询只能显示一个表中的数据．