mysql - the value doesn't show on the sp

Question

I tried a simple app for testing but doesn't get the value on my table. what I am doing wrong?

I want just put a value in a variable (not default) and then from the variable put it on the table.

like so

CREATE DEFINER=`mysql`@`localhost` 
    PROCEDURE `calcularferiado`(
        in StartDate date,
        in EndDate date,
        in Duration int)
BEGIN
    DECLARE DateDiff1 Int;

    SELECT DateDiff1 = DATEDIFF(StartDate,EndDate);

    INSERT INTO my_schema.feriados(inicio,fin,activo)
    values (StartDate, EndDate, DateDiff1);
END
        

Answer 1

 SEt DateDiff1 = DATEDIFF(StartDate,EndDate);

not

SElect DateDiff1 = DATEDIFF(StartDate,EndDate);