等值连接,自然连接,自连接分别是什么意思啊?
等值连接示范:
select * from sales s join titles t on s.title_id=t.title_id join publishers p on t.pub_id=p.pub_id
在结果集中显示冗余列数据
自然连接示范:
select t.title,p.pub_name from titles t join publishers p on t.pub_id=p.pub_id
自连接示范:
select t1.title,t2.title,t1.price from titles t1 join titles t2
on t1.price=t2.price where t1.price=2.99