How to create a user-defined function in postgresql?

@kadin 

To create a user-defined function in PostgreSQL, you can use the following syntax:

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CREATE OR REPLACE FUNCTION function_name (input_parameters)
RETURNS return_type AS
$$
DECLARE
    -- declare variables here if needed
BEGIN
    -- function logic goes here
END;
$$
LANGUAGE plpgsql;

Here's a breakdown of the syntax:

1. CREATE OR REPLACE FUNCTION: This is the statement that creates a new function or replaces an existing one with the same name.
2. function_name: This is the name of the function you want to create.
3. (input_parameters): This is where you list the input parameters for the function. You can have multiple parameters separated by commas.
4. RETURNS return_type: This specifies the data type that the function will return.
5. $$ ... $$: This is called the dollar-quoted string constant, which is used to enclose the function body. It allows you to use single quotes within the function body without escaping them.
6. DECLARE: This is where you can declare variables that will be used within the function.
7. BEGIN ... END;: This is where you write the logic of the function. This is where you can perform calculations, queries, and other operations.
8. LANGUAGE plpgsql: This specifies the language in which the function is written. In this case, it is plpgsql, which is the procedural language of PostgreSQL.

Once you have written the function definition, you can call the function like any other built-in function in PostgreSQL by using its name and passing the required parameters.