SELECT ...
  FROM UCHRFL A
  JOIN SVFLEGRUL B ON B.ID=A.SVFL
  LEFT JOIN DOLYAUSTKAPEGRUL C ON C.ID=A.DOLYAUSTKAP
WHERE ...

\r\n...\r\nLEFT JOIN DOLYAUSTKAPEGRUL C ON C.ID=A.DOLYAUSTKAP AND C.NOMINSTOIM IS NOT NULL\r\n

CREATE TABLE T1 (ID  INTEGER);


CREATE TABLE T2 (ID  INTEGER,
                 V   INTEGER);

COMMIT WORK;

INSERT INTO T1 (ID) VALUES (1);
INSERT INTO T1 (ID) VALUES (2);
INSERT INTO T1 (ID) VALUES (3);
INSERT INTO T1 (ID) VALUES (4);

COMMIT WORK;

INSERT INTO T2 (ID, V) VALUES (1, NULL);
INSERT INTO T2 (ID, V) VALUES (1, 10);
INSERT INTO T2 (ID, V) VALUES (2, 20);
INSERT INTO T2 (ID, V) VALUES (3, 30);

COMMIT WORK;

SELECT T1.*, T2.V
FROM T1
LEFT JOIN T2 ON (T2.id=T1.id) AND (T2.V IS NOT NULL )