题目链接：​​点击打开链接​​

题目大意：略。

解题思路：解决方案(2)，注：​​COUNT(student)​​​ 不能直接在 ​​WHERE​​​ 子句中使用，这里将其重命名为 ​​num​​。

AC 代码

-- 解决方案(1)
SELECT rs.class
FROM (SELECT DISTINCT student, class FROM courses) rs
GROUP BY rs.class
HAVING COUNT(1) >= 5

-- 解决方案(2)
SELECT
    class
FROM
    (SELECT
        class, COUNT(DISTINCT student) AS num
    FROM
        courses
    GROUP BY class) AS temp_table
WHERE
    num >= 5;

-- 解决方案(3)
SELECT
    class
FROM
    courses
GROUP BY class
HAVING COUNT(DISTINCT student) >= 5;