Using Expose library to write select statements with inner and outer joins

gistfile1.kt

object Users : Table() {
    val id = varchar("id", length = 10).id() // PKColumn<String, Users>
    val name = varchar("name", length = 50) // Column<String, Users>
    val requiredCityId = integer("required_city_id").references(Cities.id) // FKColumn<Int, Users>
    val optionalCityId = integer("optional_city_id").references(Cities.id).optional() // FKOptionColumn<Int, Users>

    val all = id + name + requiredCityId + optionalCityId // Template4<Users, String, Int, Int?> Select template
    val values = id + name + requiredCityId + optionalCityId // Template4<Users, String, Int, Int?> Insert template
}

object Cities : Table() {
    val id = integer("id").id().generated() // GeneratedPKColumn<Int, Cities>
    val name = varchar("name", 50) // Column<String, Cities>

    val all = id + name // Template2<Cities, Int, String> Select template
    val values = name // Column<String, Cities>
}

(Users.name + Users.requiredCityId * Cities.name) forEach {
    val (userName, cityName) = it // String, String
    println("$userName's required city is $cityName")
}

// SQL: SELECT Users.name, Cities.name FROM Users INNER JOIN Cities ON Users.required_city_id = Cities.id

(Users.name + Users.optionalCityId * Cities.name) forEach {
    val (userName, cityName) = it // String, String?
    if (cityName != null) {
        println("$userName's optional city is $cityName")
    } else {
        println("$userName has no optional city")
    }
}

// SQL: SELECT Users.id, Users.name, Cities.id, Cities.name FROM Users LEFT JOIN Cities ON Users.optional_city_id = Cities.id