SELECT stu.StuId,stu.StuName,
BM.Score AS BM,Marx.Score AS Marx,UML.Score AS UML,DB.Score AS DB,
(IFNULL(BM.Score,0)+IFNULL(Marx.Score,0)+IFNULL(UML.Score,0)+IFNULL(DB.Score,0))/
(SUM(CASE WHEN IFNULL(BM.Score,0)=0 THEN 0 ELSE 1 END)+
SUM(CASE WHEN IFNULL(Marx.Score,0)=0 THEN 0 ELSE 1 END)+
SUM(CASE WHEN IFNULL(UML.Score,0)=0 THEN 0 ELSE 1 END)+
SUM(CASE WHEN IFNULL(DB.Score,0)=0 THEN 0 ELSE 1 END)) AS AvgScore
FROM student stu
LEFT JOIN sc AS BM ON BM.SId=stu.StuId AND BM.S_CId=‘001‘
LEFT JOIN sc AS Marx ON Marx.SId=stu.StuId AND Marx.S_CId=‘002‘
LEFT JOIN sc AS UML ON UML.SId=stu.StuId AND UML.S_CId=‘003‘
LEFT JOIN sc AS DB ON DB.SId=stu.StuId AND DB.S_CId=‘004‘
GROUP BY stu.StuId
ORDER BY AvgScore DESC
LIMIT 2,4;