sql之LEFT JOIN(左连接)、RIGHT JOIN(右连接)、INNER JOIN(内连接)
2017-09-01 11:32
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表数据如下:
![](https://oscdn.geek-share.com/Uploads/Images/Content/201709/6a854fdc20f3f4f01a8d548be5357a4d)
1.LEFT JOIN(左连接)
select * fromuser u LEFT JOIN dept d on d.userId = u.userId
![](https://oscdn.geek-share.com/Uploads/Images/Content/201709/f0abf4a10867897c1cb524ebd4cf93f4)
个人理解:左连接时,以左边的数据表为基础,关联查询,右边的表查询不到到则匹配为null,匹配到则有值。
2.RIGHT JOIN(右连接)
select * fromuser u right JOIN dept d on d.userId = u.userId
![](https://oscdn.geek-share.com/Uploads/Images/Content/201709/3b706af6340766da7c8337149e12f65c)
个人理解:右连接时,以右边的数据表为基础,关联查询,左边的表查询不到到则匹配为null,匹配到则有值。
3.INNER JOIN(内连接)
select * fromuser u INNER JOIN dept d on d.userId = u.userId
![](https://oscdn.geek-share.com/Uploads/Images/Content/201709/24b759562175cdaf8119f61baebc6a05)
个人理解:内连接时,关联查询,匹配到则有值。
1.LEFT JOIN(左连接)
select * fromuser u LEFT JOIN dept d on d.userId = u.userId
个人理解:左连接时,以左边的数据表为基础,关联查询,右边的表查询不到到则匹配为null,匹配到则有值。
2.RIGHT JOIN(右连接)
select * fromuser u right JOIN dept d on d.userId = u.userId
个人理解:右连接时,以右边的数据表为基础,关联查询,左边的表查询不到到则匹配为null,匹配到则有值。
3.INNER JOIN(内连接)
select * fromuser u INNER JOIN dept d on d.userId = u.userId
个人理解:内连接时,关联查询,匹配到则有值。
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