How can we write user-defined functions (UDF) in my SQL server?

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How can we write user-defined functions (UDF) in my SQL server?

There are three types of user-defined functions(UDF) in SQL server
1)Scalar function
2)Inline function
3)Multi statement Table-valued function


1)Scalar function:-
->Scalar UDFs return a single value.
->Scalar UDFs built-in functions such as  GETDATE(),or OBJECT_NAME(), which returns single value,date or integer. The value returned by a Scalar UDF can be based on the parameters passed

Syntax:-

Create Functions  function-name(Parameters)
Returns returns-type
AS
BEGIN
      Statements 1
      Statements 2
      Statements 3
       .
       .
       Statements n
END

Example:-

Create Functions  GetEmployee(@DepartmentID int)
Returns varchar(50)
AS
BEGIN
     RETURN (SELECT Name FROM Employee WHERE       DepartmentID=@DepartmentID )
     END


2)Inline function:-
->Inline UDFs return single rows or multiple rows and can contain a single SELECT statement. Because in-line UDF is limited to a single SELECT statement.

Syntax:-

Create Functions  function-name(Parameters)
Returns returns-type
AS
BEGIN
     RETURN


Example:-

Create Functions  GetEmployee(@DepartmentID int)
Returns varchar(50)
AS
BEGIN
     RETURN 
SELECT Name FROM Employee WHERE  DepartmentID=@DepartmentID 

3)Multi statement Table-valued function:-
->Multi-statements UDF can contain any number of statements that populated the table variable to be returned.


Syntax:-

Create Functions  function-name(Parameters)
Returns @TableName TABLE
(
Column_1  Datatype,
Column_2  Datatype,
.
Column_n  Datatype
)
AS
BEGIN
Statements 1
                  Statements 2
                  Statements 3
                  .
                  .
                 Statements n
                 RETURN
END

Example:-


Create Functions  GETaverage(@Name varchar(50))
Returns  @Marks TABLE
(
Name varchar(50),
Class varchar(50),
Average Decimal(4,2)
)
AS
BEGIN
DECLARE @Avg DECIMAL(4,2)
DECLARE @Rno INT 
INSERT INTO @Marks (Name)VALUES(@Name)
SELECT @Rno=Rno FROM Student WHERE Name=@Name 
SELECT @Avg=(Subject1+Subject2+Subject3)/3 FROM Subjects WHERE Rno=@Rno

UPDATE @Marks SET
Subject1=(SELECT Subject1 FROM Subjects WHERE Rno=@Rno),
Subject2=(SELECT Subject2 FROM Subjects WHERE Rno=@Rno),
Subject3=(SELECT Subject3 FROM Subjects WHERE Rno=@Rno)
Average=@Avg
WHERE Name=@Name
                  
                 RETURN
END