Pass table as parameter into sql server UDF

sql

udfs

sql-server

performance-optimization

byAlex Kataev·Nov 29, 2024

To make a User Defined Function (UDF) works with TABLE data, SQL Server offers table-valued parameters (TVPs). First, define a user-defined table type that matches your data schema:

CREATE TYPE TableType AS TABLE (Col1 INT, Col2 VARCHAR(100));

Incorporate the defined type into your function as a parameter. Mark it as READONLY:

CREATE FUNCTION GetTableData(@TableVar TableType READONLY)
RETURNS TABLE
AS
RETURN SELECT * FROM @TableVar;

Using it in practice looks this way:

DECLARE @Data TableType;
INSERT INTO @Data VALUES (1, 'A'), (2, 'B');
SELECT * FROM GetTableData(@Data);

This method allows for robust data manipulation in a UDF.

Extending UDF functionality

UDFs in SQL Server offer a broad array of options, well beyond using table-valued parameters (TVPs). They allow endowment of SQL with custom operations making data manipulations more efficient and effective.

Using FOR XML PATH to create a CSV list

To stitch together TAB values into a CSV list inside a UDF, FOR XML PATH can be a gamechanger:

CREATE FUNCTION GetCsv(@TableVar TableType READONLY)
RETURNS VARCHAR(MAX) 
AS
BEGIN
  DECLARE @Csv VARCHAR(MAX);
  SELECT @Csv = COALESCE(@Csv + ',', '') + Col2 
  --"My CSV chain, it's off the chain!" 😆
  FROM (SELECT DISTINCT Col2 FROM @TableVar) AS DistinctValues
  RETURN @Csv;
END

This crafty function enables converting distinct column values into a CSV list.

Handling NULLs and Duplicate Values

Performing a SELECT DISTINCT before passing a table to a UDF can help eradicate duplicates or nulls. By using the COALESCE function, null values can be handled more gracefully.

Dealing with complex strings with XQuery and concatenation

To build complex strings or XML snippets from table data, SQL Server has XQuery and STUFF function coupled with FOR XML PATH. These tools help in making complex concatenations efficiently.

Stored Procedures Vs UDFs

For operations requiring a more elaborate input/output process, a stored procedure with output parameters might be a better fit compared to scalar UDFs. Temporary execution tables and transact-SQL are the real deal in stored procedures for such operations.

Guarding Against SQL Injections

When using dynamic SQL in a UDF, ensure SQL Injection is kept at bay by either sanitizing inputs or using parameterized queries.

Adapting to Your SQL Environment

Different versions of SQL Server may have different capabilities or functions. Always remember to modify untested code snippets to match with your SQL environment and thoroughly test them before deploying them to a production environment.

A Matter of Performance

Passing large tables into UDFs can sometimes have performance implications. Implement efficient indexing and query optimization techniques to ensure smooth operation.

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