练习题：
链表案例
1.查询成绩表显示如下：
一、
SELECT
score.sid,
student.sname,
student.gender,
course.cname,
score.number
FROM
score,
student,
course
WHERE
score.student_id = student.sid
AND score.corse_id = course.cid;
二、
SELECT
score.sid,
student.sname,
student.gender,
course.cname,
score.number
FROM
score
LEFT JOIN student ON score.student_id = student.sid
LEFT JOIN course ON score.corse_id = course.cid;
组合与链表查询：
2.查询每个学生的平均成绩并显示【学号、姓名、平均成绩】：、
一、
SELECT
score.sid AS 学号,
student.sname AS 姓名,
avg( score.number ) AS 平均成绩
FROM
score
LEFT JOIN student ON score.sid = student.sid
GROUP BY
score.sid
二、
SELECT
学号,
student.sname AS 姓名,平均成绩
FROM
( SELECT sid AS 学号, avg( number ) AS 平均成绩 FROM score GROUP BY sid ) AS T
LEFT JOIN student ON T.学号 = student.sid