Please help explain, multi-watch, little brother I do not understand, thank you

Please help to explain, multi-linked check, I do not understand, thank you

SQL Code

Please help to explain, multi-linked check, I do not understand, thank you

------Solution--------------------

SQL code

select * FROM (select Bookid,count (BookID) A s degree from Tb_borrow Group by BookID) as Borr join (select B.*,c.name as Bookcasename,p.pubname,t.typ  Ename from Tb_bookinfo B left joins Tb_bookcase C on B.bookcase=c.id joins Tb_publishing p on B.ISBN=P.ISBN join Tb_booktype T on B.typeid=t.id where b.del=0) as book on Borr.bookid=book.id ORDER by borr.degree desc limit ten 
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