Can I Pass Tables as Parameters to SQL Server UDFs?

Passing Tables as Parameters to SQL Server UDFs

In SQL Server, it is possible to pass tables as parameters to scalar user-defined functions (UDFs). However, this feature is not limited to all table types.

Allowed Table Types

According to Microsoft documentation, all data types are permitted for UDF parameters, except for the timestamp data type. Consequently, user-defined table types can be employed as parameters.

Creating User-Defined Table Types

To illustrate, consider the following table type:

CREATE TYPE TableType AS TABLE (LocationName VARCHAR(50))
GO

UDF Example

Now, let's define a UDF that accepts a parameter of the TableType type:

CREATE FUNCTION Example( @TableName TableType READONLY)
RETURNS VARCHAR(50)
AS
BEGIN
    DECLARE @name VARCHAR(50)

    SELECT TOP 1 @name = LocationName FROM @TableName
    RETURN @name
END

Usage

An example of usage:

DECLARE @myTable TableType
INSERT INTO @myTable(LocationName) VALUES('aaa')
SELECT * FROM @myTable

SELECT dbo.Example(@myTable)

Optional Restriction to Single-Column Tables

If you wish to limit the parameter to tables with a single column, you can modify the UDF definition to include the following check:

CREATE FUNCTION Example( @TableName TableType READONLY)
RETURNS VARCHAR(50)
AS
BEGIN
    IF (SELECT COUNT(*) FROM INFORMATION_SCHEMA.COLUMNS WHERE TABLE_NAME = @TableName AND TABLE_SCHEMA = 'dbo') > 1
        RETURN ERROR(1, 1, 'Invalid parameter: table must have a single column.');

    DECLARE @name VARCHAR(50)

    SELECT TOP 1 @name = LocationName FROM @TableName
    RETURN @name
END

Populating Variables from Existing Tables

To pass data from an existing table to the UDF parameter, you can use a variable:

DECLARE @myTable TableType

INSERT INTO @myTable(field_name)
SELECT field_name_2 FROM my_other_table

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