CREATE TABLE PEOPLE (
    ID             INTEGER NOT NULL,
    FIO            VARCHAR(255),
    MAIN_PHONE_ID  INTEGER /* Вот он*/
);


CREATE TABLE PHONE (
    ID         INTEGER NOT NULL,
    PEOPLE_ID  INTEGER,
    NMBR       VARCHAR(40) /* Значение номера */
);

INSERT INTO PEOPLE (ID, FIO, MAIN_PHONE_ID) VALUES (1, 'Иванов Иван Иванович', 1);
INSERT INTO PEOPLE (ID, FIO, MAIN_PHONE_ID) VALUES (2, 'Петров Петр Петрович', 3);
INSERT INTO PEOPLE (ID, FIO, MAIN_PHONE_ID) VALUES (3, 'Васильев Василий Васильевич', NULL);

INSERT INTO PHONE (ID, PEOPLE_ID, NMBR) VALUES (1, 1, '222-22-2222-22');
INSERT INTO PHONE (ID, PEOPLE_ID, NMBR) VALUES (2, 1, '333-33-3333-33');
INSERT INTO PHONE (ID, PEOPLE_ID, NMBR) VALUES (3, 2, '444-44-4444-44');
INSERT INTO PHONE (ID, PEOPLE_ID, NMBR) VALUES (4, 2, '555-55-5555-55');
INSERT INTO PHONE (ID, PEOPLE_ID, NMBR) VALUES (5, 3, '666-66-6666-66');
INSERT INTO PHONE (ID, PEOPLE_ID, NMBR) VALUES (6, 3, '777-77-7777-77');

ALTER TABLE PHONE ADD CONSTRAINT UNQ_PHONE_PEOPLE UNIQUE (PEOPLE_ID, ID);


ALTER TABLE PEOPLE ADD CONSTRAINT PK_PEOPLE PRIMARY KEY (ID);
ALTER TABLE PHONE ADD CONSTRAINT PK_PHONE PRIMARY KEY (ID);


ALTER TABLE PEOPLE ADD CONSTRAINT FK_PEOPLE_MAIN_PHONE FOREIGN KEY (ID, MAIN_PHONE_ID) REFERENCES PHONE (PEOPLE_ID, ID);
ALT

select P.ID, P.FIO, PH.NMBR as MAIN_PHONE
from PEOPLE P
left join PHONE PH on PH.ID = P.MAIN_PHONE_ID   

select PH.ID, PH.NMBR, iif(P.ID is not null, 'Main', null) IS_MAIN_PHONE
from PHONE PH
left join PEOPLE P on P.MAIN_PHONE_ID = PH.ID
where PH.PEOPLE_ID = :PEOPLE_ID