建立用户自定义函数，可以自由的在一个Transaction SQL 中直接使用，就像一个系统函数一样在你的查询语句中
直接使用。
CREATE FUNCTION
语法
CREATE  FUNCTION [ owner_name. ] function_name
    ( [ { @parameter_name [AS] scalar_parameter_data_type [ = default ] } [ ,...n ] ] )

RETURNS scalar_return_data_type

[ WITH < function_option> [ [,] ...n] ]

[ AS ]

BEGIN
    function_body
    RETURN scalar_expression
END

Inline Table-valued Functions

CREATE FUNCTION [ owner_name. ] function_name
    ( [ { @parameter_name [AS] scalar_parameter_data_type [ = default ] } [ ,...n ] ] )

RETURNS TABLE

[ WITH < function_option > [ [,] ...n ] ]

[ AS ]

RETURN [ ( ] select-stmt [ ) ]

Multi-statement Table-valued Functions

CREATE FUNCTION [ owner_name. ] function_name
    ( [ { @parameter_name [AS] scalar_parameter_data_type [ = default ] } [ ,...n ] ] )

RETURNS @return_variable TABLE < table_type_definition >

[ WITH < function_option > [ [,] ...n ] ]

[ AS ]

BEGIN
    function_body
    RETURN
END

< function_option > ::=
    { ENCRYPTION | SCHEMABINDING }

< table_type_definition > :: =
    ( { column_definition | table_constraint } [ ,...n ] )

示例

--创建函数
create  function getValue()
returns int
as
begin
return (select convert(int,Value) AS Value from Test where  [Name]='Test')
end
go

select * from Test2 where Value=dbo.getValue()

go

create  function getTabletest(@t1 datetime,@t2 datetime)
returns table
as
 return( select t1.Field1 ,t2.Field1,t2.Qty,t3.Field1,t3.Field1
 from Table1 t1 inner join  Table2 t2 on t1.ID=t2.ID inner join Table3 t3 on t2.JID=t3.JID where t1.Date>=@t1 and t1.Date<@t2)

go

select * from dbo.getTabletest('1999/1/1','2001/2/1')