[SQL] cast 与 convert 都在什么情况下使用

CONVERT(data_type,expression[,style])

说明:
此样式一般在时间类型(datetime,smalldatetime)与字符串类型(nchar,nvarchar,char,varchar)
相互转换的时候才用到.

例子:
SELECT CONVERT(varchar(),getdate(),) now
结果为
now
---------------------------------------
//

/////////////////////////////////////////////////////////////////////////////////////

style数字在转换时间时的含义如下

-------------------------------------------------------------------------------------------------
Style(2位表示年份) | Style(4位表示年份) | 输入输出格式
-------------------------------------------------------------------------------------------------
- |  or  | mon dd yyyy hh:miAM(或PM)
-------------------------------------------------------------------------------------------------
 |  | mm/dd/yy
-------------------------------------------------------------------------------------------------
 |  | yy-mm-dd
-------------------------------------------------------------------------------------------------
 |  | dd/mm/yy
-------------------------------------------------------------------------------------------------
 |  | dd-mm-yy
-------------------------------------------------------------------------------------------------
 |  | dd-mm-yy
-------------------------------------------------------------------------------------------------
 |  | dd mon yy
-------------------------------------------------------------------------------------------------
 |  | mon dd,yy
-------------------------------------------------------------------------------------------------
 |  | hh:mm:ss
-------------------------------------------------------------------------------------------------
- |  or  | mon dd yyyy hh:mi:ss:mmmmAM(或PM)
-------------------------------------------------------------------------------------------------
 |  | mm-dd-yy
-------------------------------------------------------------------------------------------------
 |  | yy/mm/dd
-------------------------------------------------------------------------------------------------
 |  | yymmdd
-------------------------------------------------------------------------------------------------
- |  or  | dd mon yyyy hh:mi:ss:mmm(24小时制)
-------------------------------------------------------------------------------------------------
 |  | hh:mi:ss:mmm(24小时制)
-------------------------------------------------------------------------------------------------
- |  or  | yyyy-mm-dd hh:mi:ss(24小时制)
-------------------------------------------------------------------------------------------------
- |  or  | yyyy-mm-dd hh:mi:ss:mmm(24小时制) 

例如当前时间为：-- ::

我要取其中的：--

select convert(char(),'2005-9-12 13:20:00',)

CAST ( expression AS data_type )
将某种数据类型的表达式显式转换为另一种数据类型
declare @a int
cast(@a as varchar())--从而连接动态语句