Not allowed to return a result ? stored procedure mysql

CREATE TABLE ZAMOWIENIA( id_zamowienia INT PRIMARY KEY NOT NULL,
    id_uzytkownika INT, kwota_zamowienia DECIMAL(10,2));

INSERT INTO
    ZAMOWIENIA VALUES (1,1,20), (2,4 ,5), (3,3,100), (4,1,300),
    (5,2,80), (6, 1,150);

SELECT * FROM ZAMOWIENIA;

select
    count(id_uzytkownika) from ZAMOWIENIA WHERE id_uzytkownika=1;

DROP
    FUNCTION AccountType;

DELIMITER //
CREATE FUNCTION AccountType(
    in_id_uzytkownika INT) RETURNS VARCHAR(20) DETERMINISTIC 
BEGIN
    DECLARE account_type VARCHAR(20); 
    DECLARE in_id_uzytkownika INT;
    DECLARE zamowienia INT;

    select count(id_uzytkownika)  AS zamowienia 
    from ZAMOWIENIA 
    WHERE id_uzytkownika=in_id_uzytkownika; 

    select zamowienia from zamowienia;

    IF zamowienia >10 THEN      
        SET account_type = 'BRONZE';
    END IF;      
    IF zamowienia >30 THEN      
        SET account_type = 'SILVER';
    END IF;     
    IF zamowienia >60 THEN      
        SET account_type = 'GOLD';
    END IF;

    return account_type; 
END // DELIMITER ;

Answer

If you want to set a variable to the result of a SELECT query, you need to use SELECT ... INTO variable, not SELECT ... AS variable. Your function is trying to return the result of the SELECT query because it doesn’t store the result into a variable.

Or you can use SET statement, which is simpler when you’re just setting a single variable.

So replace the two SELECT statements with:

SET zamowienia = (SELECT COUNT(*) FROM from ZAMOWIENIA 
    WHERE id_uzytkownika=in_id_uzytkownika); 

Also, you shouldn’t have DECLARE statements for the function parameters. Remove the line

DECLARE in_id_uzytkownika INT;