User-defined Functions in SQL

The term "user-defined function" (UDF) in relation to SQL refers to a special function that may be written and utilized inside SQL statements. It is a section of code that carries out a certain task and gives the caller a result. A UDF can be used in SELECT, WHERE, and other queries to do various calculations and data manipulations.

There are two types of user-defined functions in SQL:

1. Scalar Functions: A scalar function depends on input parameters to return a single value. It can take one or more input parameters and apply a particular operation on them, such as adding two numbers together, changing a text to uppercase letters, or figuring out an individual's age from their birthdate.

2. Table-Valued Functions: A table is the result set of a table-valued function. To produce a table of results, it can take one or more input parameters and apply a certain operation to them. Table-valued functions are useful for many different tasks, including filtering and merging data from different tables, running calculations on huge datasets, and creating reports.

They make it possible to package sophisticated logic and reuse it across several queries. Additionally, they can make it simpler to maintain and change SQL code over time and increase the readability of the code. A named collection of SQL statements that can be called from other SQL statements is known as a user-defined function in SQL. A user-defined function can return a scalar value or a table and accept zero or more input arguments.

1. Here is an example of a scalar function that calculates the square of a number: CREATE FUNCTION square(@num INT) RETURNS INT AS BEGIN RETURN @num * @num; END;

2. Here is an example of a table-valued function that returns all employees in a specific department: CREATE FUNCTION get_employees(@dept_id INT) RETURNS TABLE AS RETURN ( SELECT * FROM employees WHERE department_id = @dept_id );

There are several reasons to use user-defined functions in SQL:

1. Code Reusability: The time and effort required to write and maintain SQL code can be reduced by using user-defined functions, which can be developed once and reused in a number of SQL queries or stored procedures.

2. Encapsulation: The SQL code may be simpler to comprehend and maintain if user-defined functions are used to combine complex logic and calculations into a single function.

3. Performance: By enabling difficult calculations to be done only once and then reused numerous times within a query, user-defined functions can enhance query efficiency.

4. Abstraction: The use of user-defined functions can abstract away repetitious or difficult SQL code, making it simpler for developers to use and comprehend.

5. Consistency: By guaranteeing that the same logic is applied consistently throughout the programm, user-defined functions can enforce consistency in SQL queries and stored procedures.

Here are the basic steps for using a user-defined function in SQL:

1. Create the Function: Use the CREATE FUNCTION statement to define the user-defined function. The function has to be given a name, any necessary input parameters, and a return type. Here is a simple scalar function that adds two numbers as an example: CREATE FUNCTION add_numbers (@num1 INT, @num2 INT) RETURNS INT AS BEGIN RETURN @num1 + @num2; END;

2. Call the Function: Use the function's name and any other necessary input parameters to call it. The function can be used in a stored procedure, a SELECT, INSERT, UPDATE, or DELETE statement. Here's an example of calling the add_numbers function:

SELECT dbo.add_numbers(10, 5) AS result;

This will return the result of adding 10 and 5, which is 15.

3. Use the Function: Use the function anywhere that would use a scalar value or a table in SQL. For example, use the function in a WHERE clause to filter results based on the function's output: SELECT * FROM employees WHERE dbo.get_salary(employee_id) > 50000;

This will return all employees whose salary, as determined by the get_salary function, is greater than 50,000.

Here is an exercise to practice using user-defined functions in SQL:

Think about a database that includes data on employees, such as their salary. Making a user-defined function that determines each employee's bonus based on salary is the next step. This is how the bonus should be calculated:

• Employees with a salary less than 50,000 get a bonus equal to 10% of their salary.
• Employees with a salary between 50,000 and 100,000 get a bonus equal to 15% of their salary.
• Employees with a salary greater than 100,000 get a bonus equal to 20% of their salary.

Create the user-defined function calculate_bonus, which accepts only one input argument (@salary) and returns the calculated bonus. Then create a SELECT statement that extracts all of the employees' information from the database, including their pay and bonuses.

Here's a possible solution to this exercise: -- Create the user-defined function CREATE FUNCTION calculate_bonus (@salary DECIMAL(10,2)) RETURNS DECIMAL(10,2) AS BEGIN DECLARE @bonus DECIMAL(10,2); IF @salary < 50000 SET @bonus = @salary * 0.1; ELSE IF @salary >= 50000 AND @salary <= 100000 SET @bonus = @salary * 0.15; ELSE SET @bonus = @salary * 0.2; RETURN @bonus; END; GO -- Retrieve all employees with their salaries and bonuses SELECT employee_id, first_name, last_name, salary, dbo.calculate_bonus(salary) AS bonus FROM employees;

This will create the calculate_bonus function and then retrieve all employees from the database, along with their salaries and bonuses, as calculated by the function.

Here is another exercise to practice using user-defined functions in SQL:

Think of a database that has data on products, such as their prices and quantities. Make a user-defined function that determines the overall income for a certain product while accounting for any necessary discounts. The discounts should be computed using the formula below:

• If the product's quantity is less than 10, no discount applies.
• If the product's quantity is between 10 and 50, a 5% discount applies.
• If the product's quantity is between 51 and 100, a 10% discount applies.
• If the product's quantity is greater than 100, a 15% discount applies.

Create a user-defined function called calculate_revenue that accepts the two inputs @price and @quantity and returns the product's total revenue, less any relevant discounts. Afterward, create a SELECT statement that extracts all of the products from the database along with their costs, sales totals, and quantities.

Here's a possible solution to this exercise: -- Create the user-defined function CREATE FUNCTION calculate_revenue (@price DECIMAL(10,2), @quantity INT) RETURNS DECIMAL(10,2) AS BEGIN DECLARE @discount DECIMAL(10,2); DECLARE @total DECIMAL(10,2); IF @quantity < 10 SET @discount = 0; ELSE IF @quantity >= 10 AND @quantity <= 50 SET @discount = 0.05; ELSE IF @quantity >= 51 AND @quantity <= 100 SET @discount = 0.1; ELSE SET @discount = 0.15; SET @total = @price * @quantity * (1 - @discount); RETURN @total; END; GO -- Retrieve all products with their prices, quantities, and total revenues SELECT product_id, product_name, price, quantity, dbo.calculate_revenue(price, quantity) AS revenue FROM products;

This will create the calculate_revenue function and then retrieve all products from the database, along with their prices, quantities, and total revenues, as calculated by the function.