city = {                                                                       /* Города */
    id : integer
    name : string : not null
}, {
    id : primary key
    name : key
}

sales = {
    id : integer                                                               /* Продажи */
    city_id : integer
    day : datetime : not null
    value : integer : not null
}, {
    id : primary key
    city_id : key
    day : key
}, {
    city.id <- city_id
}