题解 | 找出每个学校GPA最低的同学

SELECT 
    u.device_id,
    u.university,
    u.gpa
FROM 
    user_profile u
JOIN (
    SELECT 
        university,
        MIN(gpa) AS min_gpa
    FROM 
        user_profile
    GROUP BY 
        university
) AS min_gpa_subquery 
ON 
    u.university = min_gpa_subquery.university 
    AND u.gpa = min_gpa_subquery.min_gpa
ORDER BY 
    u.university ASC;

1. SELECT u.device_id, u.university, u.gpa：选择要返回的列，分别是设备 ID、大学名称和 GPA。
2. FROM user_profile u：指定数据来源表为user_profile，并给它取别名u。
3. JOIN (SELECT university, MIN(gpa) AS min_gpa FROM user_profile GROUP BY university) AS min_gpa_subquery ON u.university = min_gpa_subquery.university AND u.gpa = min_gpa_subquery.min_gpa：子查询部分SELECT university, MIN(gpa) AS min_gpa FROM user_profile GROUP BY university：按大学分组，找出每个大学中的最低 GPA。主查询通过JOIN将user_profile表（别名u）与子查询（别名min_gpa_subquery）连接起来，连接条件是大学名称相同且主表中的 GPA 等于子查询中该大学的最低 GPA。
4. ORDER BY u.university ASC：按大学名称升序排列结果。