create table student(birthday varchar(30));

insert into student values('-1');

insert into student values('2013-01-01');

--报错 select * from student where isdate(birthday)=1 and cast(birthday as datetime) = '2013-01-01';

--正确

select * from student where birthday='2013-01-01' and cast(birthday as datetime) = '2013-01-01';

这是为什么呢？