User-defined functions (UDFs) in HSQLDB allow developers to extend the functionality of the database by creating custom functions that can be used in SQL queries. These functions are defined by the user and can encapsulate complex logic, calculations, or operations that are not directly supported by built-in SQL functions. Here's an overview of the concept of UDFs in HSQLDB:
Purpose :* UDFs allow developers to create custom functions to perform specific tasks or calculations within SQL queries.
* They provide a way to encapsulate complex logic or calculations into reusable functions, improving code readability, maintainability, and performance.
Types of UDFs :HSQLDB supports two types of user-defined functions :
* Scalar Functions : Scalar functions return a single value based on the input parameters. They can be used anywhere a value is expected in a SQL query, such as in SELECT statements, WHERE clauses, or expressions.
* Aggregate Functions : Aggregate functions operate on a set of values and return a single aggregated result. They are typically used with the GROUP BY clause to perform calculations across multiple rows.
Syntax :The syntax for defining a UDF in HSQLDB depends on the type of function:
* Scalar Function : CREATE FUNCTION function_name(parameter_list) RETURNS return_type AS function_body
* Aggregate Function : CREATE AGGREGATE FUNCTION function_name(parameter_list) RETURNS return_type
Usage :* Once a UDF is defined, it can be used in SQL queries just like built-in functions.
* Scalar UDFs can be used in SELECT statements, WHERE clauses, ORDER BY clauses, and other places where expressions are allowed.
* Aggregate UDFs can be used with the GROUP BY clause to perform calculations on grouped data.
Example :Here's an example of defining and using a scalar UDF in HSQLDB:
CREATE FUNCTION calculate_discount(price DOUBLE, discount_rate DOUBLE) RETURNS DOUBLE AS
BEGIN
RETURN price * (1 - discount_rate);
END;
SELECT product_name, calculate_discount(price, 0.1) AS discounted_price
FROM Products;?
