SELECT  mo.id, mo.season,
        @r AS `LAG(month) OVER (PARTITION BY season ORDER BY id)`,
        (@r := month) AS month
FROM    (
        SELECT  m.*
        FROM (SELECT @_season = NULL, @s := NULL) vars,
                t_month m
        ORDER BY
                season, id
        ) mo
WHERE  (CASE WHEN @_season IS NULL OR @_season <> season THEN @r := NULL ELSE NULL END IS NULL)
        AND (@_season := season) IS NOT NULL;