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Table-valued User-defined Functions

User-defined functions that return a table data type can be powerful alternatives to views. These functions are referred to as table-valued functions. A table-valued user-defined function can be used where table or view expressions are allowed in Transact-SQL queries. While views are limited to a single SELECT statement, user-defined functions can contain additional statements that allow more powerful logic than is possible in views.

A table-valued user-defined function can also replace stored procedures that return a single result set. The table returned by a user-defined function can be referenced in the FROM clause of a Transact-SQL statement, but stored procedures that return result sets cannot.

In a table-valued user-defined function:

  • The RETURNS clause defines a local return variable name for the table returned by the function. The RETURNS clause also defines the format of the table. The scope of the local return variable name is local within the function.
  • The Transact-SQL statements in the function body build and insert rows into the return variable defined by the RETURNS clause.
  • When a RETURN statement is executed, the rows inserted into the variable are returned as the tabular output of the function. The RETURN statement cannot have an argument.

No Transact-SQL statements in a table-valued function can return a result set directly to a user. The only information the function can return to the user is the table returned by the function.

ms191165.note(en-US,SQL.90).gifNote:
The text in row table option is automatically set to 256 for a table returned by a user-defined function. This cannot be changed. The READTEXT, WRITETEXT, and UPDATETEXT statements cannot be used to read or write parts of any text, ntext, or image columns in the table. For more information, see In-Row Data.

Example

The following example creates the dbo.ufnGetContactInformation function and demonstrates the components of the table-valued function. In this function, the local return variable name is @retContactInformation. Statements in the function body insert rows into this variable to build the table result returned by the function.

USE AdventureWorks;
GO
IF OBJECT_ID(N'dbo.ufnGetContactInformation', N'TF') IS NOT NULL
    DROP FUNCTION dbo.ufnGetContactInformation;
GO
CREATE FUNCTION dbo.ufnGetContactInformation(@ContactID int)
RETURNS @retContactInformation TABLE 
(
    -- Columns returned by the function
    ContactID int PRIMARY KEY NOT NULL, 
    FirstName nvarchar(50) NULL, 
    LastName nvarchar(50) NULL, 
    JobTitle nvarchar(50) NULL, 
    ContactType nvarchar(50) NULL
)
AS 
-- Returns the first name, last name, job title, and contact type for the specified contact.
BEGIN
    DECLARE 
        @FirstName nvarchar(50), 
        @LastName nvarchar(50), 
        @JobTitle nvarchar(50), 
        @ContactType nvarchar(50);
    -- Get common contact information
    SELECT 
        @ContactID = ContactID, 
        @FirstName = FirstName, 
        @LastName = LastName
    FROM Person.Contact 
    WHERE ContactID = @ContactID;
    SELECT @JobTitle = 
        CASE 
            -- Check for employee
            WHEN EXISTS(SELECT * FROM HumanResources.Employee AS e 
                WHERE e.ContactID = @ContactID) 
                THEN (SELECT Title 
                    FROM HumanResources.Employee 
                    WHERE ContactID = @ContactID)
            -- Check for vendor
            WHEN EXISTS(SELECT * FROM Purchasing.VendorContact AS vc 
                    INNER JOIN Person.ContactType AS ct 
                    ON vc.ContactTypeID = ct.ContactTypeID 
                WHERE vc.ContactID = @ContactID) 
                THEN (SELECT ct.Name 
                    FROM Purchasing.VendorContact AS vc 
                        INNER JOIN Person.ContactType AS ct 
                        ON vc.ContactTypeID = ct.ContactTypeID 
                    WHERE vc.ContactID = @ContactID)
            -- Check for store
            WHEN EXISTS(SELECT * FROM Sales.StoreContact AS sc 
                    INNER JOIN Person.ContactType AS ct 
                    ON sc.ContactTypeID = ct.ContactTypeID 
                WHERE sc.ContactID = @ContactID) 
                THEN (SELECT ct.Name 
                    FROM Sales.StoreContact AS sc 
                        INNER JOIN Person.ContactType AS ct 
                        ON sc.ContactTypeID = ct.ContactTypeID 
                    WHERE ContactID = @ContactID)
            ELSE NULL 
        END;
    SET @ContactType = 
        CASE 
            -- Check for employee
            WHEN EXISTS(SELECT * FROM HumanResources.Employee AS e 
                WHERE e.ContactID = @ContactID) 
                THEN 'Employee'
            -- Check for vendor
            WHEN EXISTS(SELECT * FROM Purchasing.VendorContact AS vc 
                    INNER JOIN Person.ContactType AS ct 
                    ON vc.ContactTypeID = ct.ContactTypeID 
                WHERE vc.ContactID = @ContactID) 
                THEN 'Vendor Contact'
            -- Check for store
            WHEN EXISTS(SELECT * FROM Sales.StoreContact AS sc 
                    INNER JOIN Person.ContactType AS ct 
                    ON sc.ContactTypeID = ct.ContactTypeID 
                WHERE sc.ContactID = @ContactID) 
                THEN 'Store Contact'
            -- Check for individual consumer
            WHEN EXISTS(SELECT * FROM Sales.Individual AS i 
                WHERE i.ContactID = @ContactID) 
                THEN 'Consumer'
        END;
    -- Return the information to the caller
    IF @ContactID IS NOT NULL 
    BEGIN
        INSERT @retContactInformation
        SELECT @ContactID, @FirstName, @LastName, @JobTitle, @ContactType;
    END;
    RETURN;
END;
GO

The following example uses the table-valued function dbo.ufnGetContactInformation in the FROM clause of two SELECT statements.

USE AdventureWorks;
GO
SELECT ContactID, FirstName, LastName, JobTitle, ContactType
FROM dbo.ufnGetContactInformation(2200);
GO
SELECT ContactID, FirstName, LastName, JobTitle, ContactType
FROM dbo.ufnGetContactInformation(5);
GO

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