题解 | 查询成绩

-- 考虑到有学生为参加考试导致分数为 NULL 的情况
SELECT
    count(*)
FROM
    (
        SELECT
            St.sId,
            sum(IFNULL(SC.score, 0)) / count(distinct C.cname) AS avg_score
        FROM
            Student St
            LEFT JOIN SC USING (sId)
            LEFT JOIN Course  C USING (cId) 
        GROUP BY
            sId
        HAVING
            sum(IFNULL(score, 0)) / 3 > 60
    ) t