MySQL基础--链接查询

MySQL基础–链接查询

MySQL内连接。取两个表都有的数据—交集！

给数据表取名字-------on表示条件。找两个表的#交集
select s.name,c.name from students as s inner join classes as c on s.cls_id = c.id;
有时候我们把某张表名字搞错了，需要修改：
rename table 表1 to 表2;
交换位置：
select c.name,s.name from students as s inner join classes as c on s.cls_id = c.id;
按照班级进行排序：
select c.name,s.name from students as s inner join classes as c on s.cls_id = c.id order by c.name desc;
对两个字段进行排序
select c.name,s.name from students as s inner join classes as c on s.cls_id = c.id order by c.name desc s.id asc;

MySQL外连接—分为左连接和右连接

左连接，以左边的表为基准，取东西，取不到的显示为NULL
select * from students as s left join classes as c on s.cls_id=c.id;

右连接，以右边的表为基准，取东西，，，相当于左连接的两张表交换位置
select * from students as s right join classes as c on s.cls_id=c.id; 用的很少！！！

查询没有对应班级信息的学生
select * from students as s left join classes as c on s.cls_id=c.id having c.id is null;

多表查询时候用链接查询