Какой побочный эффект возникает при выполнении кода BEGIN; SET TRANSACTION ISOLATION LEVEL READ COMMITTED; START TRANSACTION; SELECT progress INTO @current_progress FROM courses WHERE course_id = 101; SET @current_progress = @current_progress + 60; UPDATE courses SET progress = @current_progress WHERE course_id = 101; COMMIT; END; BEGIN; SET TRANSACTION ISOLATION LEVEL READ COMMITTED; START TRANSACTION; SELECT progress INTO @current_progress FROM courses WHERE course_id = 101; SET @current_progress = @current_progress + 40; UPDATE courses SET progress = @current_progress WHERE course_id = 101; COMMIT; END;