Cast(字段名 as 转换的类型 )，其中类型可以为：
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
例如表table1
date
2015-11-03 15:31:26
select cast(date as signed) as date from table1;
结果如下：
date
20151103153126
select cast(date as char) as date from table1;
结果如下：
date
2015-11-03 15:31:26
select cast(date as datetime) as date from table1;
结果如下：
date
2015-11-03 15:31:26
select cast(date as date) as date from table1;
结果如下：
date
2015-11-03
select cast(date as time) as date from table1;
结果如下：
date
15:31:26
这里date对应日期，time对应时间
