mysql为我们提供了两个类型转换函数：CAST和CONVERT，现成的东西我们怎能放过？
BINARY[(N)]
CHAR[(N)]
DATE
DATETIME
DECIMAL
SIGNED [INTEGER]
TIME
UNSIGNED [INTEGER]
例子：
--使用CAST将varchar转换成int类型排序
select server_id from cardserver where game_id = 1 order by CAST(server_id as SIGNED) desc limit 10; --使用CONVERT将varchar转换成int类型排序
select server_id from cardserver where game_id = 1 order by CONVERT(server_id,SIGNED) desc limit 10
参考 http://blog.163.com/jzq_520/blog/static/119311262201171192914788/