Mysql在比较两种不同数据类型时，第一步是将他们转化为同一种类型，然后在比较。那么Date和String在比较的时候，一定是把String转化为Date吗？答案是Yes. http://dev.mysql.com/doc/refman/5.0/en/type-conversion.html If one of the arguments is a TIMESTAMP or DATETIME column and the other argument is a constant, the constant is converted to a timestamp before the comparison is performed.
Mysql关于Date类型的文档 http://dev.mysql.com/doc/refman/5.5/en/datetime.html
其中有这样一段话：invalid dates such as '2004-04-31' are converted to '0000-00-00' and a warning is generated. With strict mode enabled, invalid dates generate an error.
Mysql在比较两种不同数据类型时，第一步是将他们转化为同一种类型，然后在比较。那么Date和String在比较的时候，一定是把String转化为Date吗？答案是Yes.
http://dev.mysql.com/doc/refman/5.0/en/type-conversion.html
If one of the arguments is a TIMESTAMP or DATETIME column and the other argument is a constant, the constant is converted to a timestamp before the comparison is performed.简单举个例子，如果是用字符串比较，"2004-04-31"这个string应该比2004-01-01这个date来得大，但是4-31是一个invalid的日期（4月是小月），会被转化成"0000-00-00"，所以2004-01-01 (日期) > "2014-04-31"。
Mysql关于Date类型的文档
http://dev.mysql.com/doc/refman/5.5/en/datetime.html
其中有这样一段话：
invalid dates such as '2004-04-31' are converted to '0000-00-00' and a warning is generated. With strict mode enabled, invalid dates generate an error.