SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;

BEGIN TRANSACTION
    SELECT
        SUM(Cena)
    FROM
        Wycieczki
-- STEP 1 RUN ABOVE

    INSERT INTO 
        Bilety 
    VALUES
        (2, 'Honolulu', 1, 2000);
-- STEP 3 RUN BETWEEN 1 AND 3

COMMIT TRANSACTION