MySQL － isnull usage

SELECT book.code, book.bookname, book.author, book.publisher, ifnull(cc.testtime,0)
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SELECT book.code, book.bookname, book.author, book.publisher, if(cc.testtime is null,0,cc.testtime)
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SELECT book.code, book.bookname, book.author, book.publisher, if(isnull(cc.testtime),0,cc.testtime)
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 as borrow_times
FROM book LEFT JOIN (select loan.code as testcode ,count(loan.stuid) as testtime
from loan
group by loan.code)  AS cc ON book.code=cc.testcode
ORDER BY book.code;

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Result：
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code	bookname	author	publisher	borrow_times
c001	Harry	Charlie	charlie	2
c002	The Ring	Andy	Tonli	1
c003	Konan	Dowa	Tonli	0