The SQL CREATE FUNCTION statement is used to define a new user-defined function (UDF) in a database. A function in SQL is a set of SQL statements that perform a specific task and return a single value. Functions help in encapsulating a set of logic that can be reused in various parts of SQL queries, enhancing code modularity and maintainability.


Here’s the basic syntax for creating a function:

CREATE FUNCTION function_name (
   parameter1 datatype, 
   parameter2 datatype, 
RETURNS return_datatype
    -- SQL statements to define the function logic

    RETURN expression; 
    -- Return statement indicating the result of the function

Let’s break down the components of the CREATE FUNCTION statement:

function_name: This is the name of the function you are creating. It should follow the rules for naming conventions in the database system you are using.

(parameter1 datatype, parameter2 datatype, …): These are the input parameters that the function will accept. Each parameter should have a name and a data type.

RETURNS return_datatype: Specifies the data type of the value that the function will return.

AS: Begins the body of the function.

BEGIN and END: Enclose the set of SQL statements that make up the function’s logic. The statements are executed when the function is called.

RETURN expression: This statement specifies the value that the function will return. The data type of the returned value should match the return_datatype specified earlier.


Here’s a simple example of a SQL function that calculates the square of a number:

CREATE FUNCTION CalculateSquare (input_number INT)
    DECLARE result INT;
    SET result = input_number * input_number;
    RETURN result;

You can then use this function in your SQL queries like this:

SELECT CalculateSquare(5) AS SquareResult;

This will return the result of the function, which is 25 in this case.

Keep in mind that the syntax and features of user-defined functions may vary between different database management systems (DBMS), such as MySQL, PostgreSQL, SQL Server, etc. Be sure to refer to the documentation of the specific DBMS you are using for more details and variations.