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Create user-defined functions (Database Engine)

  • Artikkeli

Applies to: SQL Server Azure SQL Database Azure SQL Managed Instance Azure Synapse Analytics Analytics Platform System (PDW)

This article describes how to create a user-defined function (UDF) in SQL Server by using Transact-SQL.

Limitations and restrictions

  • User-defined functions can't be used to perform actions that modify the database state.

  • User-defined functions can't contain an OUTPUT INTO clause that has a table as its target.

  • User-defined functions can't 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 doesn't support TRY...CATCH, @ERROR or RAISERROR.

  • User-defined functions can't call a stored procedure, but can call an extended stored procedure.

  • User-defined functions can't make use of dynamic SQL or temp tables. Table variables are allowed.

  • SET statements aren't allowed in a user-defined function.

  • The FOR XML clause isn't 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 don't 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
    • END CONVERSATION
    • GET CONVERSATION GROUP
    • MOVE CONVERSATION
    • RECEIVE
    • SEND

Permissions

Requires 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.

Scalar function examples

Scalar function (scalar UDF)

The following example creates a multi-statement scalar function (scalar UDF) in the AdventureWorks2022 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.

Table-valued function examples

Inline table-valued function (TVF)

The following example creates an inline table-valued function (TVF) in the AdventureWorks2022 database. The function takes one input parameter, a customer (store) ID, and returns the columns ProductID, 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);

Multi-statement table-valued function (MSTVF)

The following example creates a multi-statement table-valued function (MSTVF) in the AdventureWorks2022 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.

Best practices

If a user-defined function (UDF) isn't 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's invoked. We recommend that you implement one of the following methods to ensure that the function doesn't become outdated because of changes to its underlying objects:

  • Specify the WITH SCHEMABINDING clause when you're creating the UDF. This ensures that the objects referenced in the function definition can't 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 doesn't access data, specify the SCHEMABINDING option to 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 an 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 use 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 isn't 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 INSERT or UPDATE statement succeeds.

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