discount = {                                                                   /* Скидки */
    id : integer
    name : string : not null
    relative_value : float : not null
    date_from, date_to : datetime : not null
}, {
    id : primary key
    date_from : key
}

discount_group = {                                                             /* Скидки <+> группы */
    discount_id : integer
    group_id : integer
}, {
    discount_id, group_id : unique key
    group_id, discount_id : unique key
}, {
    discount.id <- discount_id
    group.id <- group_id
}