SQL Basics – UDF | User Defined Functions – Scalar, Table Valued (TVF), MultiStatement (MTVF)

UDF or User Defined Functions are a set or batch of code where one can apply any SQL logic and return a single scalar value or a record set.

According to MS BOL UDFs are the subroutines made up of one or more Transact-SQL statements that can be used to encapsulate code for reuse. These reusable subroutines can be used as:
– In TSQL SELECT statements at column level.
– To create parametrized view or improve the functionality of in indexed view.
– To define a column and CHECK constraints while creating a table.
– To replace a stored procedures and views.
– Join complex logic with a table where a stored procedure fails.
– Faster execution like Stored procedures, reduce compliation cost by caching the execution query plans.

Apart from the benefits UDF’s has certain limitations:
– Can not modify any database objects, limited to update table variables only.
– Can not contain the new OUTPUT clause.
– Can only call extended stored procedures, no other procedures.
– Can not define TRY-CATCH block.
– Some built-in functions are not allowed here, like:GETDATE(), because GETDATE is non-deterministic as its value changes every time it is called. On the other hand DATEADD() is allowed as it is deterministic, because it will return same result when called with same argument values.

A UDF can take 0 or upto 1024 parameters and returns either a scalar value or a table record set depending on its type.
SQL Server supports mainly 3 types of UDFs:
1. Scalar function
2. Inline table-valued function
3. Multistatement table-valued function

1. Scalar function: Returns a single value of any datatype except text, ntext, image, cursor & timestamp.

-- Example:
--// Create Scalar UDF [dbo].[ufn_GetContactOrders]
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE FUNCTION [dbo].[ufn_GetContactOrders](@ContactID int)
RETURNS varchar(500)
AS
BEGIN
	DECLARE @Orders varchar(500)

	SELECT @Orders = COALESCE(@Orders + ', ', '') + CAST(SalesOrderID as varchar(10))
	FROM Sales.SalesOrderHeader
	WHERE ContactID = @ContactID

	RETURN (@Orders)
END

--// Usage:
-- Used at COLUMN level with SELECT
SELECT ContactID, dbo.ufn_GetContactOrders(ContactID) FROM Person.Contact
WHERE ContactID between 100 and 105 -- Output below

-- Used while defining a computed column while creating a table.
CREATE TABLE tempCustOrders (CustID int, Orders as (dbo.ufn_GetContactOrders(CustID)))

INSERT INTO tempCustOrders (CustID)
SELECT ContactID FROM Person.Contact
WHERE ContactID between 100 and 105

SELECT * FROM tempCustOrders -- Output below

DROP TABLE tempCustOrders

Output of both the selects above:
ContactID	OrdersCSV
100		51702, 57021, 63139, 69398
101		47431, 48369, 49528, 50744, 53589, 59017, 65279, 71899
102		43874, 44519, 46989, 48013, 49130, 50274, 51807, 57113, 63162, 69495
103		43691, 44315, 45072, 45811, 46663, 47715, 48787, 49887, 51144, 55310, 61247, 67318
104		43866, 44511, 45295, 46052, 46973, 47998, 49112, 50215, 51723, 57109, 63158, 69420
105		NULL

Note: If this was a temp(#) table then the function also needs to be created in tempdb, cause the temp table belongs to tempdb. The tables in function should also have the database name prefixed, i.e. [AdventureWorks].[Sales].[SalesOrderHeader]

2. Inline table-valued function: Returns a table i.e. a record-set. The function body contains just a single TSQL statement, which results to a record-set and is returned from here.

-- Example:
--// Create Inline table-valued UDF [dbo].[ufn_itv_GetContactSales]
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE FUNCTION [dbo].[ufn_itv_GetContactSales](@ContactID int)
RETURNS TABLE
AS
RETURN (
	SELECT h.[ContactID], h.[SalesOrderID], p.[ProductID], p.[Name], h.[OrderDate], h.[DueDate],
	h.[ShipDate], h.[TotalDue], h.[Status], h.[SalesPersonID]
	FROM Sales.SalesOrderHeader AS h
	JOIN Sales.SalesOrderDetail AS d ON d.SalesOrderID = h.SalesOrderID
	JOIN Production.Product AS p ON p.ProductID = d.ProductID
	WHERE ContactID = @ContactID )

--// Usage:
SELECT * FROM ufn_itv_GetContactSales(100)

3. Multistatement table-valued function: Also returns a table (record-set) but can contain multiple TSQL statements or scripts and is defined in BEGIN END block. The final set of rows are then returned from here.

-- Example:
--// Create Multistatement table-valued UDF [dbo].[ufn_mtv_GetContactSales]
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE FUNCTION [dbo].[ufn_mtv_GetContactSales](@ContactID int)
RETURNS @retSalesInfo TABLE (
	[ContactID] INT NOT NULL,
	[SalesOrderID] INT NULL,
	[ProductID] INT NULL,
	[Name] NVARCHAR(50) NULL,
	[OrderDate] DATETIME NULL,
	[DueDate] DATETIME NULL,
	[ShipDate] DATETIME NULL,
	[TotalDue] MONEY NULL,
	[Status] TINYINT NULL,
	[SalesPersonID] INT NULL)
AS
BEGIN
	IF @ContactID IS NOT NULL
	BEGIN
		INSERT @retSalesInfo
		SELECT h.[ContactID], h.[SalesOrderID], p.[ProductID], p.[Name], h.[OrderDate], h.[DueDate],
			   h.[ShipDate], h.[TotalDue], h.[Status], h.[SalesPersonID]
		FROM Sales.SalesOrderHeader AS h
		JOIN Sales.SalesOrderDetail AS d ON d.SalesOrderID = h.SalesOrderID
		JOIN Production.Product AS p ON p.ProductID = d.ProductID
		WHERE ContactID = @ContactID
	END
	-- Return the recordsets
	RETURN
END

--// Usage:
SELECT * FROM ufn_mtv_GetContactSales(100)

— Output:

TVF & MVF output

More MSDN & MS BOL links:
http://msdn.microsoft.com/en-us/library/aa175085(SQL.80).aspx
http://msdn.microsoft.com/en-us/library/aa214363(SQL.80).aspx
http://msdn.microsoft.com/en-us/library/ms186755.aspx
http://msdn.microsoft.com/en-us/library/ms191007.aspx
http://msdn.microsoft.com/en-us/magazine/cc164062.aspx