left join,rignt join,inner join的区分
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外链接:
left join(left outer join):左外连接(匹配左表所有行,如果左表的某行在右表没有匹配行,则在相关联的右表要查询的列用空值代替)
right join (right outer join) : 右外连接(匹配右表所有行,如果右表的某行在左表没有匹配行,则在相关联的左表要查询的列用空值代替)
内连接:
inner join:内连接(左右表匹配ID行)
如:学生表和成绩表
Student
id name
001 a
002 b
Score
id scores
001 80
003 90
左外连接:
select * from Student stu left join Score sco on stu.id=sco.id
id name id scores
001 a 001 80
002 b
右外链接:
select * from Student stu right join Score sco on stu.id=sco.id
id name id scores
001 a 001 80
003 90
内连接:
select * from Student (as) stu inner join Score (as) sco on stu.id=sco.idselect
id name id scores
001 a 001 80
0 0
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