我想将NULL转话为''再插入,应该怎么办?

[handsome0916]

insert into bb select str(cc.id) as id,replace(num,NULL,'') as num from cc where (id<10 or id>10)

replace(num,NULL,'')这样不太行,还有没有什么函数类似于SQL SERVER的num=isnull(num,'')方法的?