SQL左外连接和内连接有什么区别?

SQL左外连接和内连接有什么区别?急!!!!
2024-11-19 19:44:06
推荐回答(3个)
回答1:

有区别
左外连接是返回主表的所有信息,如果从表没有主表信息显示为空
而内连接是以从表为主如没数据那么就不显示
举个例子
如有2张表 student grade
student
有sid,sname两列
有3条数据 1,呵呵 2,嘿嘿 3,嘻嘻
grade
有score,sid
有2条数据 90,1 80,2
比如我们要查哪个人考了多少分呢么左连接就是
呵呵 90
嘿嘿 80
嘻嘻 null
内连接就是
呵呵 90
嘿嘿 80

回答2:

内连接

我们可以知道至少是两个表

比如

on a.id=b.id

那么这两个id都不允许为空
换言之

也就是他们都不能为空!

而左连接的意思是:以第一个表

比如
on a.id=b.id

这个时候的第一个表也就是a

以a.id为标准

比如
a.id一共有20条数据

如果b表中没有数据那么以null来填充!

共同学习!

我想通过我这么介绍

右连接以及完全连接就不用说了吧

回答3:

外连接分左连接和右连接,
左连接以左表为基表,左表全部显示包括空值,右表关联展示,右连接反之。
内连接可以理解为左连接和右连接的交集。

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