【Mysql学习日志】基本命令(二)

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;

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;

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

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