1.数据类型转换：cast(字段名 as 转换的类型)
常用类型如下：
CHAR[(N)] 字符型
DATE 日期型
DATETIME 日期和时间型
DECIMAL float型
SIGNED int
TIME 时间型
示例：date(类型是日期)字段的值是 2015-11-03 15:31:26
cast(date as signed) - 20151103153126
cast(date as char) - 2015-11-03 15:31:26
cast(date as datetime) - 2015-11-03 15:31:26
cast(date as date) - 2015-11-03
cast(date as time) - 15:31:26
