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<>mysql中的外连接和内连接
内连接
内连接:指连接结果仅包含符合连接条件的行,参与连接的两个表都应该符合连接条件。
INNER JOIN关键字表示在表中存在至少一个匹配时
取交集 查询两张表 之间的关联关系 商品表和用户表之间的关联关系 SELECT * FROM USER INNER JOIN orderlist ON
orderlist.uid = user.id;
内连接格式
查询用户 姓名 年龄 和订单编号 select u.name, u,age, u,number from user u inner join
orderlist oon o.uid = u.id;
外连接
外连接:连接结果不仅包含符合连接条件的行同时也包含自身不符合条件的行。包括左外连接、右外连接和全外连接。
左连接 右连接
*
左连接 以 左表为主 左表的数据必须全部显示
*
右连接 以 右表为主的数据全部显示 如果没有对应的数据 用null来补
左连接 即
左边内容全部显示 用右表和左表进行匹配
最后展示左左表全部内容以及和右表交集的内容
left join: 包含左表的所有行,对应的右表行可能为空。
right join: 包含右表的所有行,对应的左表行可能为空。
左连接语法格式
查询所有的用户信息 以及用户对应的订单信息 select u.*, o.number from user u left outer join
orderlist oon o.uid = u.id
其中 left join 和 left outer join作用是一样的