在MySQL中，请看以下表：
Books
------
BookId
BookName
AuthorId
SubjectId
PopularityRating（该书的受欢迎程度，从 1 到 10）
Language（如 French、English、German 等）
/////////////////////////////////////////////////
Subjects
---------
SubjectId
Subject（如 History、Geography、Mathematics 等）
/////////////////////////////////////////////////
Authors
--------
AuthorId
AuthorName
Country
哪个查询可以确定每个主题有多少本书的受欢迎程度超过7？( )
select subject,count(*) as Books from books,subjects where books.popularityrating > 7
select subject,count(*) as Books from books,subjects where books.authorid=subjects.authorid and books.popularityrating > 7 group by subjects.subject
select subject,count(*) as Books from books,subjects where books.subjectid=subjects.subjectid and books.popularityrating = 7 group by subjects.subject
select subject,count(*) as Books from books,subjects where books.subjectid=subjects.subjectid and books.popularityrating > 7 group by subjects.subject
