User-defined functions (UDFs) in SQL are custom functions that users create to extend the capabilities of SQL beyond what is provided by standard built-in functions. UDFs allow developers to encapsulate complex logic and reusable operations into a single function, improving the maintainability of SQL code. These functions can be invoked in SQL statements just like any built-in function, allowing for greater flexibility in querying and data manipulation.
There are primarily two types of UDFs: scalar and table-valued functions. Scalar UDFs return a single value, which can be of any data type. For example, a scalar UDF could be created to calculate the sales tax on a given amount by taking the amount as an input parameter and returning the calculated tax. Table-valued UDFs, on the other hand, return a table. They are useful for complex queries that require outputting multiple rows and columns. For instance, a table-valued function could return a summary of sales grouped by product categories.
When creating a UDF, developers need to define the function's name, its parameters, and the return type. A typical creation syntax looks like this for a scalar function:
CREATE FUNCTION CalculateTax(@Amount DECIMAL(10, 2))
RETURNS DECIMAL(10, 2)
AS
BEGIN
RETURN @Amount * 0.07; -- Example tax rate of 7%
END;
Once defined, this function can be used in SELECT statements or other SQL commands. Using UDFs can significantly enhance code readability and reuse, making it easier for developers to manage and operate with complex data logic in their database systems.