5、查询所有同学的学生编号、学生姓名、选课总数、所有课程的总成绩
SELECT st.s_id,st.s_name,c,s
FROM student st
LEFT JOIN
(SELECT sc.s_id,COUNT(sc.c_id) c,SUM(sc.s_score) s
FROM score sc
GROUP BY
sc.s_id) d ON st.s_id=d.s_id
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