SQL查询--连接查询

      使用的数据库：https://blog.csdn.net/weixin_44953902/article/details/100169739
      当查询结果的列来源于多张表时，需要将多张表连接成一个大的数据集，再选择合适的列返回。
      mysql支持三种类型的连接查询，分别为：

内连接（inner join … in）

  -- select ... from 表A inner join 表B;
  select * from students inner join classes;

-- 查询 有能够对应班级的学生以及班级信息
  select * from students inner join classes on students.cls_id=classes.id;

-- 按照要求显示姓名、班级
  select students.name,classes.name from students inner join classes on students.cls_id=classes.id;

--给数据表起名字
  select s.name,c.name from students as s inner join classes as c on s.cls_id=c.id;

-- 在以上的查询中,将班级名单显示在第1列
  select c.name,s.* from students as s inner join classes as c on s.cls_id=c.id;

-- 查询 有能够对应班级的学生以及班级信息,按照班级进行排序
  select c.name,s.* from students as s inner join classes as c on s.cls_id=c.id order by c.name;

-- 班级相同时,按照学生的id及逆行排序
  select c.name,s.* from students as s inner join classes as c on s.cls_id=c.id order by c.name,s.id;

左连接（left join）

-- 查询每位学生对应的班级信息
  select * from students as s left join classes as c on s.cls_id=c.id;

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

right join … on

一般用的很少：将数据表名字互换位置，用left join 完成