mysql - SQL 外连接

Question

三张表courses,registration,students

学生列

名字,姓氏,studentid,major,admitdate,graddate,gender,dob

注册中的列

courseid,studentid

类(class)中的列

类(class)编号，类(class)名称，学分

我需要修改的select语句

select lastname as 'Last Name',sum(credits) as 'Credits Registered For' from students   as s
inner join registration as r on s.studentid = r.studentid
inner join courses as c on c.coursenumber = c.courseid
group by last name;

实验室的问题是...修改之前的查询以显示所有学生，即使他们尚未注册类(class)。你应该有 14 行。未注册的学生将在输出中显示 NULL。

我知道这需要某种外部连接，但我没有完全掌握这些连接我已经阅读了这里和其他网站上的多篇文章，但似乎无法弄清楚。

Answer 1

使用LEFT JOIN

select  lastname as 'Last Name',
        sum(credits) as 'Credits Registered For' 
from    students   as s
        LEFT join registration as r on s.studentid = r.studentid
        LEFT join courses as c on c.coursenumber = r.courseid
group by last name;