left join、right join、inner join原理

sql的left join 、right join 、inner join之间的区别

   例：<br />        ![圖片.png](https://cdn.nlark.com/yuque/0/2020/png/1329724/1606877216322-57147d7f-a977-4728-8986-bb6bab62da6a.png#align=left&display=inline&height=56&margin=%5Bobject%20Object%5D&name=%E5%9C%96%E7%89%87.png&originHeight=112&originWidth=314&size=2091&status=done&style=none&width=157)![圖片.png](https://cdn.nlark.com/yuque/0/2020/png/1329724/1606880134812-1b10e91e-188e-4814-9526-9ad44b12d7f3.png#align=left&display=inline&height=64&margin=%5Bobject%20Object%5D&name=%E5%9C%96%E7%89%87.png&originHeight=128&originWidth=304&size=1852&status=done&style=none&width=152)<br />　　1.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<br />       ![圖片.png](https://cdn.nlark.com/yuque/0/2020/png/1329724/1606877277770-785a86c4-1a25-4ef5-8536-4bcc39ce45dc.png#align=left&display=inline&height=116&margin=%5Bobject%20Object%5D&name=%E5%9C%96%E7%89%87.png&originHeight=231&originWidth=942&size=6716&status=done&style=none&width=471)<br />        结果说明:<br />left join是以A表的记录为基础的,A可以看成左表,B可以看成右表,left join是以左表为准的.

　　2.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

结果说明:
仔细观察一下,就会发现,和left join的结果刚好相反,这次是以右表(B)为基础的,A表不足的地方用NULL填充.
　　3.inner join(等值连接) 只返回两个表中联结字段相等的行
person表例新增一条数据