Mysql之inner join,left join,right join
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Mysql之inner join,left join,right join详解
inner join(等值连接):只返回两个表中联结字段相等的行;
left join(左联接):返回包括左表中的所有记录和右表中联结字段相等的记录;
right join(右联接):返回包括右表中的所有记录和左表中联结字段相等的记录。
比如我们有xs、cj两个表
xs表 cj表
--------------- ----------------------
id name id score
1 张三 1 96
2 李四 2 80
3 86
Sql代码
1 SELECT * FROM `xs` INNER JOIN `cj` ON xs.id = cj.id
返回
------------------------
id name id score
1 张三 1 96
2 李四 2 80
-----------------------
Sql代码
2 SELECT * FROM `xs` LEFT JOIN `cj` ON xs.id = cj.id
返回
------------------------
id name id score
1 张三 1 96
2 李四 2 80
-----------------------
Sql代码
3 SELECT * FROM `xs` RIGHT JOIN `cj` ON xs.id = cj.id
返回
id name id score
1 张三 1 96
2 李四 2 80
NULL NULL 3 86
其中还有inner join还有另外一种写法,两者是等价的,都是等值连接
Sql代码
4 SELECT * FROM `xs`,`cj` WHERE xs.id = cj.id
转载:http://blog.csdn.net/wangzff/article/details/7238566
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