Create User-defined Functions (Database Engine)
This topic describes how to create a user-defined function (UDF) in SQL Server by using Transact-SQL.
Before You Begin
Limitations and restrictions
User-defined functions cannot be used to perform actions that modify the database state.
User-defined functions cannot contain an
OUTPUT INTOclause that has a table as its target.
User-defined functions can not return multiple result sets. Use a stored procedure if you need to return multiple result sets.
Error handling is restricted in a user-defined function. A UDF does not support
User-defined functions cannot call a stored procedure, but can call an extended stored procedure.
User-defined functions cannot make use of dynamic SQL or temp tables. Table variables are allowed.
SETstatements are not allowed in a user-defined function.
FOR XMLclause is not allowed.
User-defined functions can be nested; that is, one user-defined function can call another. The nesting level is incremented when the called function starts execution, and decremented when the called function finishes execution. User-defined functions can be nested up to 32 levels. Exceeding the maximum levels of nesting causes the whole calling function chain to fail. Any reference to managed code from a Transact-SQL user-defined function counts as one level against the 32-level nesting limit. Methods invoked from within managed code do not count against this limit.
The following Service Broker statements cannot be included in the definition of a Transact-SQL user-defined function:
BEGIN DIALOG CONVERSATION
GET CONVERSATION GROUP
CREATE FUNCTION permission in the database and
ALTER permission on the schema in which the function is being created. If the function specifies a user-defined type, requires
EXECUTE permission on the type.
The following example creates a multi-statement scalar function (scalar UDF) in the AdventureWorks2012 database. The function takes one input value, a
ProductID, and returns a single data value, the aggregated quantity of the specified product in inventory.
IF OBJECT_ID (N'dbo.ufnGetInventoryStock', N'FN') IS NOT NULL DROP FUNCTION ufnGetInventoryStock; GO CREATE FUNCTION dbo.ufnGetInventoryStock(@ProductID int) RETURNS int AS -- Returns the stock level for the product. BEGIN DECLARE @ret int; SELECT @ret = SUM(p.Quantity) FROM Production.ProductInventory p WHERE p.ProductID = @ProductID AND p.LocationID = '6'; IF (@ret IS NULL) SET @ret = 0; RETURN @ret; END;
The following example uses the
ufnGetInventoryStock function to return the current inventory quantity for products that have a
ProductModelID between 75 and 80.
SELECT ProductModelID, Name, dbo.ufnGetInventoryStock(ProductID)AS CurrentSupply FROM Production.Product WHERE ProductModelID BETWEEN 75 and 80;
For more information and examples of scalar functions, see CREATE FUNCTION (Transact-SQL).
The following example creates an inline table-valued function (TVF) in the AdventureWorks2012 database. The function takes one input parameter, a customer (store) ID, and returns the columns
Name, and the aggregate of year-to-date sales as
YTD Total for each product sold to the store.
IF OBJECT_ID (N'Sales.ufn_SalesByStore', N'IF') IS NOT NULL DROP FUNCTION Sales.ufn_SalesByStore; GO CREATE FUNCTION Sales.ufn_SalesByStore (@storeid int) RETURNS TABLE AS RETURN ( SELECT P.ProductID, P.Name, SUM(SD.LineTotal) AS 'Total' FROM Production.Product AS P JOIN Sales.SalesOrderDetail AS SD ON SD.ProductID = P.ProductID JOIN Sales.SalesOrderHeader AS SH ON SH.SalesOrderID = SD.SalesOrderID JOIN Sales.Customer AS C ON SH.CustomerID = C.CustomerID WHERE C.StoreID = @storeid GROUP BY P.ProductID, P.Name );
The following example invokes the function and specifies customer ID 602.
SELECT * FROM Sales.ufn_SalesByStore (602);
The following example creates a multi-statement table-valued function (MSTVF) in the AdventureWorks2012 database. The function takes a single input parameter, an
EmployeeID and returns a list of all the employees who report to the specified employee directly or indirectly. The function is then invoked specifying employee ID 109.
IF OBJECT_ID (N'dbo.ufn_FindReports', N'TF') IS NOT NULL DROP FUNCTION dbo.ufn_FindReports; GO CREATE FUNCTION dbo.ufn_FindReports (@InEmpID INTEGER) RETURNS @retFindReports TABLE ( EmployeeID int primary key NOT NULL, FirstName nvarchar(255) NOT NULL, LastName nvarchar(255) NOT NULL, JobTitle nvarchar(50) NOT NULL, RecursionLevel int NOT NULL ) --Returns a result set that lists all the employees who report to the --specific employee directly or indirectly.*/ AS BEGIN WITH EMP_cte(EmployeeID, OrganizationNode, FirstName, LastName, JobTitle, RecursionLevel) -- CTE name and columns AS ( SELECT e.BusinessEntityID, e.OrganizationNode, p.FirstName, p.LastName, e.JobTitle, 0 -- Get the initial list of Employees for Manager n FROM HumanResources.Employee e INNER JOIN Person.Person p ON p.BusinessEntityID = e.BusinessEntityID WHERE e.BusinessEntityID = @InEmpID UNION ALL SELECT e.BusinessEntityID, e.OrganizationNode, p.FirstName, p.LastName, e.JobTitle, RecursionLevel + 1 -- Join recursive member to anchor FROM HumanResources.Employee e INNER JOIN EMP_cte ON e.OrganizationNode.GetAncestor(1) = EMP_cte.OrganizationNode INNER JOIN Person.Person p ON p.BusinessEntityID = e.BusinessEntityID ) -- copy the required columns to the result of the function INSERT @retFindReports SELECT EmployeeID, FirstName, LastName, JobTitle, RecursionLevel FROM EMP_cte RETURN END; GO
The following example invokes the function and specifies employee ID 1.
SELECT EmployeeID, FirstName, LastName, JobTitle, RecursionLevel FROM dbo.ufn_FindReports(1);
For more information and examples of inline table-valued functions (inline TVFs) and multi-statement table-valued functions (MSTVFs), see CREATE FUNCTION (Transact-SQL).
If a user-defined function (UDF) is not created with the
SCHEMABINDING clause, changes that are made to underlying objects can affect the definition of the function and produce unexpected results when it is invoked. We recommend that you implement one of the following methods to ensure that the function does not become outdated because of changes to its underlying objects:
WITH SCHEMABINDINGclause when you are creating the UDF. This ensures that the objects referenced in the function definition cannot be modified unless the function is also modified.
Execute the sp_refreshsqlmodule stored procedure after modifying any object that is specified in the definition of the UDF.
If creating a UDF that does not access data, specify the
SCHEMABINDING option. This will prevent the query optimizer from generating unnecessary spool operators for query plans involving these UDFs. For more information on spools, see Showplan Logical and Physical Operators Reference. For more information on creating a schema bound function, see Schema-bound functions.
Joining to an MSTVF in a
FROM clause is possible, but can result in poor performance. SQL Server is unable to use all the optimized techniques on some statements that can be included in a MSTVF, resulting in a suboptimal query plan. To obtain the best possible performance, whenever possible use joins between base tables instead of functions.
MSTVFs have a fixed cardinality guess of 100 starting with SQL Server 2014 (12.x), and 1 for earlier SQL Server versions.
Starting with SQL Server 2017 (14.x), optimizing an execution plan that uses MSTVFs can leverage interleaved execution, which results in using actual cardinality instead of the above heuristics.
For more information, see Interleaved execution for multi-statement table valued functions.
ANSI_WARNINGS is not honored when you pass parameters in a stored procedure, user-defined function, or when you declare and set variables in a batch statement. For example, if a variable is defined as char(3), and then set to a value larger than three characters, the data is truncated to the defined size and the
UPDATE statement succeeds.