数据库 一个数据库中有四张表，分别如下：

A.SELECT S#,AVG(score) FROM GROUP BY S# HAVING AVG(score)>60

b.SELECT S.Ss#,S.Sname,COUNT(SC.C#),SUM(SC.score) FROM Student S LEFT JOIN SC ON S.Ss#=S# GROUP BY S.S#

1) select s.Name from Student s join StuCur c on s.id=c.sid

join Course cu on cu.id=c.cid where cu.Name='自然'

2) select c.Name from Course c join StuCur sc on c.id=sc.CID

join Student s on sc.sid=s.id where s.EntranceTime between '1999-01-01' and '2012-12-31'

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