Multi-Statement Table Valued Functions in SQL Server - Part 32

we will discuss about Multi-Statement Table Valued functions.

Multi statement table valued functions are very similar to Inline Table valued functions, with a few differences. Let's look at an example, and then note the differences. 

Employees Table:


Let's write an Inline and multi-statement Table Valued functions that can return the output shown below.



Inline Table Valued function(ILTVF):
Create Function fn_ILTVF_GetEmployees()
Returns Table
as
Return (Select Id, Name, Cast(DateOfBirth as Date) as DOB
       From tblEmployees)


Multi-statement Table Valued function(MSTVF):
Create Function fn_MSTVF_GetEmployees()
Returns @Table Table (Id int, Name nvarchar(20), DOB Date)
as
Begin
Insert into @Table
Select Id, Name, Cast(DateOfBirth as Date)
From tblEmployees

Return
End

Calling the Inline Table Valued Function:
Select * from fn_ILTVF_GetEmployees()

Calling the Multi-statement Table Valued Function:
Select * from fn_MSTVF_GetEmployees()

Now let's understand the differences between Inline Table Valued functions and Multi-statement Table Valued functions
1. In an Inline Table Valued function, the RETURNS clause cannot contain the structure of the table, the function returns. Where as, with the multi-statement table valued function, we specify the structure of the table that gets returned
2. Inline Table Valued function cannot have BEGIN and END block, where as the multi-statement function can have.
3. Inline Table valued functions are better for performance, than multi-statement table valued functions. If the given task, can be achieved using an inline table valued function, always prefer to use them, over multi-statement table valued functions.
4. It's possible to update the underlying table, using an inline table valued function, but not possible using multi-statement table valued function.

Updating the underlying table using inline table valued function: 
This query will change Sam to Sam1, in the underlying table tblEmployees. When you try do the same thing with the multi-statement table valued function, you will get an error stating 'Object 'fn_MSTVF_GetEmployees' cannot be modified.'
Update fn_ILTVF_GetEmployees() set Name='Sam1' Where Id = 1

Reason for improved performance of an inline table valued function:
Internally, SQL Server treats an inline table valued function much like it would a view and treats a multi-statement table valued function similar to how it would a stored procedure.