SQL Table-Valued functions

SQL Table-Valued Function (TVF) is a user-defined function that returns a table as a result set. Unlike scalar functions that return a single value, a TVF can be used to encapsulate a complex logic that generates and returns a table of data. TVFs are particularly useful when you need to perform a set of operations on a dataset and return the result as a table, which can then be used in a query like a regular table or joined with other tables.

Here are some key characteristics and considerations for SQL Table-Valued Functions:

Types of TVFs

Inline Table-Valued Functions (Inline TVFs): These functions are similar to views and return a table variable directly. They are defined using the RETURNS TABLE clause and are created with the INLINE option.

CREATE FUNCTION dbo.InlineTVF (@param INT)
RETURNS TABLE
AS
RETURN
(
    SELECT * 
    FROM SomeTable 
    WHERE Column = @param
);

Multi-statement Table-Valued Functions (Multi-statement TVFs): These functions use the RETURNS @table_variable TABLE syntax and include a BEGIN…END block to define the logic. They are created with the MULTI_STATEMENT option.

CREATE FUNCTION dbo.MultiStatementTVF (@param INT)
RETURNS @ResultTable TABLE
(
    Column1 INT,
    Column2 VARCHAR(50)
)
AS
BEGIN
    INSERT INTO @ResultTable
    SELECT Column1, Column2 
    FROM SomeTable 
    WHERE Column = @param;
    
    RETURN;
END;

Parameters

TVFs can accept parameters, allowing you to pass values into the function to influence the logic and filter the results.

Usage in Queries

Once defined, TVFs can be used in queries like tables or views, allowing you to join them with other tables, apply filters, and include them in the SELECT, FROM, and JOIN clauses.

SELECT *
FROM dbo.InlineTVF(123) AS result
JOIN AnotherTable AS at ON result.ID = at.ID;

Performance

TVFs can have performance implications, and the choice between inline and multi-statement TVFs can impact query performance. In general, inline TVFs are often more efficient because they are expanded in the execution plan.

Reusability

TVFs promote code reusability by encapsulating complex logic into a function that can be easily called from various parts of your database.

Schema Binding

TVFs can be schema-bound, which means they are bound to the schema of the underlying tables. This can provide additional benefits such as the prevention of underlying table schema changes that could affect the TVF.

In summary, SQL Table-Valued Functions provide a powerful way to encapsulate and reuse logic that generates a set of rows as a result. They enhance code modularity, readability, and maintainability while offering a flexible way to work with sets of data in SQL queries.

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