-- 20、查询学生的总成绩并进行排名

SELECT st.s_id,st.s_name,(CASE WHEN SUM(sc.s_score) IS NULL THEN 0 ELSE SUM(sc.s_score) END) AS '总成绩' FROM student st LEFT JOIN score sc ON sc.s_id=st.s_id GROUP BY st.s_id ORDER BY SUM(sc.s_score) DESC