内外连接

最新推荐文章于 2024-08-22 05:00:00 发布

最新推荐文章于 2024-08-22 05:00:00 发布 · 94 阅读

本文通过三个具体的SQL查询语句，详细对比了内连接（INNER JOIN）、左连接（LEFT JOIN）的不同使用场景及结果差异。重点讲解如何根据需求选择合适的连接方式来获取学生课程系统与成就系统之间的关联数据。

select * from studentcoursesys a inner join dbo.achievementsys b
on a.userid='guozhaofu' and a.courseid=b.courseid and a.userid=b.userid

select * from studentcoursesys a left join dbo.achievementsys b
on a.userid='guozhaofu' and a.courseid=b.courseid and a.userid=b.userid

select * from studentcoursesys a left join dbo.achievementsys b
on a.courseid=b.courseid and a.userid=b.userid
where a.userid='guozhaofu'
--------------------------------------------------------------
分析上面三条语句的区别