[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(30),getdate(),101) now

结果为

now

---------------------------------------

09/15/2001



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



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 

-------------------------------------------------------------------------------------------------

- | 0 or 100 | mon dd yyyy hh:miAM(或PM) 

-------------------------------------------------------------------------------------------------

1 | 101 | mm/dd/yy 

-------------------------------------------------------------------------------------------------

2 | 102 | yy-mm-dd 

-------------------------------------------------------------------------------------------------

3 | 103 | dd/mm/yy 

-------------------------------------------------------------------------------------------------

4 | 104 | dd-mm-yy 

-------------------------------------------------------------------------------------------------

5 | 105 | dd-mm-yy 

-------------------------------------------------------------------------------------------------

6 | 106 | dd mon yy 

-------------------------------------------------------------------------------------------------

7 | 107 | mon dd,yy 

-------------------------------------------------------------------------------------------------

8 | 108 | hh:mm:ss 

-------------------------------------------------------------------------------------------------

- | 9 or 109 | mon dd yyyy hh:mi:ss:mmmmAM(或PM)

-------------------------------------------------------------------------------------------------

10 | 110 | mm-dd-yy 

-------------------------------------------------------------------------------------------------

11 | 111 | yy/mm/dd 

-------------------------------------------------------------------------------------------------

12 | 112 | yymmdd 

-------------------------------------------------------------------------------------------------

- | 13 or 113 | dd mon yyyy hh:mi:ss:mmm(24小时制) 

-------------------------------------------------------------------------------------------------

14 | 114 | hh:mi:ss:mmm(24小时制) 

-------------------------------------------------------------------------------------------------

- | 20 or 120 | yyyy-mm-dd hh:mi:ss(24小时制) 

-------------------------------------------------------------------------------------------------

- | 21 or 121 | yyyy-mm-dd hh:mi:ss:mmm(24小时制) 





例如当前时间为：2005-9-12 13:20:00



我要取其中的：2005-9-12



select convert(char(10),'2005-9-12 13:20:00',120)





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(10))--从而连接动态语句