内外连接

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'
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分析上面三条语句的区别