Common system functions in SQL

--1 CONVERT (data type, expression), CAST (expression as data type) Transform data type
--Convert a number to a string
SELECT CONVERT (varchar (2), +convert)--+ (varchar (2), ten ) is a connector
--the same
SELECT CAST (as varchar (2)) +cast (as varchar (2))

--Converting a string to an integral type
SELECT Convert (int, ' ten ') +convert (int, '--+ ') is a plus sign
--the same
SELECT CAST (' ten ' as int) +cast (' as int ')

--Convert the date to a character type, with the date format as follows:

With Standard

without Century (yy) (1)	century (yyyy)		Input/output (3)
-	0 or 100 (UP)	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 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 (UP)	Default + milliseconds	Mon dd yyyy hh:mi:ss:mmmAM (or PM)
10	110	USA	Ten = Mm-dd-yy MM-DD-YYYY =
11	111	JAPAN	one = Yy/mm/dd 111 = Yyyy/mm/dd
12	112	Iso	YYMMDD = YYYYMMDD =
-	113 (+)	Europe default + milliseconds	DD Mon yyyy hh:mi:ss:mmm (24h)
14	114	-	Hh:mi:ss:mmm (24h)
-	or 120 (2)	ODBC Canonical	Yyyy-mm-dd Hh:mi:ss (24h)
-	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, and 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, and 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 (+)	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), GETDATE (), 126)

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

--the same
SELECT CAST (GETDATE () as varchar (10))

--In addition, you can convert to the following format:

--2 Current_User Returns the name of the current user
SELECT Current_User

--3 Datalenfth Returns the number of bytes used to specify an expression
SELECT datalength (' China a ')
SELECT datalength (' 123433 ')

--4 HOST_NAME Returns the name of the computer to which the current user is logged on
SELECT HOST_NAME () --Computer name
SELECT host_id () --Computer ID

--5 System_user Returns the name of the user currently logged in
SELECT System_user

--6 user_name Returns the user name from the given user ID
SELECT user_name (1)

Common system functions in SQL