SQL中常用系统函数

--1 CONVERT(数据类型,表达式),CAST( 表达式 AS 数据类型) 转变数据类型
--将数字转化为字符串
SELECT CONVERT(varchar(2),12)+CONVERT(varchar(2),10) --+为连接符
--同下
SELECT CAST(12 AS varchar(2))+CAST(10 AS varchar(2))

--将字符串转化为整型
SELECT CONVERT(int,'12')+CONVERT(int,'10' ) --+为加号符
--同下
SELECT CAST('12' AS int)+CAST('10' AS int)

--将日期转化为字符型,其中的日期格式如下：

Without century (yy) (1)	With century (yyyy)	Standard	Input/Output (3)
-	0 or 100 (1,2)	Default for datetime and smalldatetime	mon dd yyyy hh:miAM (or PM)
1	101	U.S.	1 = mm/dd/yy 101 = mm/dd/yyyy
2	102	ANSI	2 = yy.mm.dd 102 = yyyy.mm.dd
3	103	British/French	3 = dd/mm/yy 103 = dd/mm/yyyy
4	104	German	4 = dd.mm.yy 104 = dd.mm.yyyy
5	105	Italian	5 = dd-mm-yy 105 = dd-mm-yyyy
6	106(1)	-	6 = dd mon yy 106 = dd mon yyyy
7	107(1)	-	7 = Mon dd, yy 107 = Mon dd, yyyy
8	108	-	hh:mi:ss
-	9 or 109 (1,2)	Default + milliseconds	mon dd yyyy hh:mi:ss:mmmAM (or PM)
10	110	USA	10 = mm-dd-yy 110 = mm-dd-yyyy
11	111	JAPAN	11 = yy/mm/dd 111 = yyyy/mm/dd
12	112	ISO	12 = yymmdd 112 = yyyymmdd
-	13 or 113 (1,2)	Europe default + milliseconds	dd mon yyyy hh:mi:ss:mmm(24h)
14	114	-	hh:mi:ss:mmm(24h)
-	20 or 120 (2)	ODBC canonical	yyyy-mm-dd hh:mi:ss(24h)
-	21 or 121 (2)	ODBC canonical (with milliseconds) default for time, date, datetime2, and datetimeoffset	yyyy-mm-dd hh:mi:ss.mmm(24h)
-	126 (4)	ISO8601	yyyy-mm-ddThh:mi:ss.mmm (no spaces) Note: When the value for milliseconds (mmm) is 0, the millisecond value is not displayed. For example, the value '2012-11-07T18:26:20.000 is displayed as '2012-11-07T18:26:20'.
-	127(6, 7)	ISO8601 with time zone Z.	yyyy-mm-ddThh:mi:ss.mmmZ (no spaces) Note: When the value for milliseconds (mmm) is 0, the milliseconds value is not displayed. For example, the value '2012-11-07T18:26:20.000 is displayed as '2012-11-07T18:26:20'.
-	130 (1,2)	Hijri (5)	dd mon yyyy hh:mi:ss:mmmAM In this style, mon represents a multi-token Hijri unicode representation of the full month's name. This value does not render correctly on a default US installation of SSMS.
-	131 (2)	Hijri (5)	dd/mm/yyyy hh:mi:ss:mmmAM

SELECT CONVERT(varchar(10),GETDATE(),126)

SELECT CONVERT(varchar(10),GETDATE(),100)

--同下
SELECT CAST(GETDATE() AS varchar(10) )

--此外还可以转换为以下格式：

--2 CURRENT_USER 返回当前用户的名字
SELECT CURRENT_USER

--3 DATALENFTH 返回用于指定表达式的字节数
SELECT DATAlENGTH('中国A')
SELECT DATALENGTH('123433')

--4 HOST_NAME 返回当前用户所登录的计算机名字
SELECT HOST_NAME() --计算机名字
SELECT HOST_ID() --计算机ID

--5 SYSTEM_USER 返回当前所登陆的用户名称
SELECT SYSTEM_USER

--6 USER_NAME 从给定的用户ID 返回用户名
SELECT USER_NAME(1)