select * from table1 
where (table1.fam='ИВАНОВ' and table1.imj='ИВАН') 
      or exists (select 'x' from table2 
                   where table2.fam='ИВАНОВ' 
                     and table2.imj = 'ИВАН' 
                     and table2.parent_id=table1.id)

DECLARE
	STR  VARCHAR2(1000) := NULL;
	
BEGIN
	IF :TABLE1.FAM IS NOT NULL THEN
		STR := STR || 'S.FAM='''||:TABLE1.FAM||''' AND ';
	END IF;	
	IF :TABLE1.IMJ IS NOT NULL THEN
		STR := STR || 'S.IMJ='''||:TABLE1.IMJ||''' AND ';
	END IF;	
	IF :TABLE1.OTCH IS NOT NULL THEN
		STR := STR || 'S.OTCH='''||:TABLE1.OTCH||')';	
	ELSE /* Здесь можно и поизящней, но пока так */
		STR := RTRIM(STR, 'AND '); 
		STR := STR || ')';
	END IF;			
	IF LENGTH(STR) > 3 /*Здесь можно и поизящней, но пока так*/ THEN
		COPY(NULL, 'TABLE1.FAM');
		COPY(NULL, 'TABLE1.IMJ');
		COPY(NULL, 'TABLE1.OTCH');		
		SET_BLOCK_PROPERTY('TABLE1', DEFAULT_WHERE, '(' || REPLACE(STR, 'S.', 'TABLE1.') || ' OR EXISTS (SELECT ''X'' FROM TABLE2 S WHERE S.PARENT_NSYST = TABLE1.NSYST AND '||STR);
	END IF;			
  END;  
END;  

SELECT *
   FROM TABLE1
  WHERE (TABLE1.FAM = 'ИВАНОВ' AND TABLE1.IMJ = 'ИВАН')
     OR EXISTS (SELECT 'X'
           FROM TABLE2 S
          WHERE S.PARENT_NSYST = TABLE1.NSYST
            AND S.FAM = 'ИВАНОВ'
            AND S.IMJ = 'ИВАН')
	AND (TABLE1.FAM = 'ИВАНОВ') AND (TABLE1.IMJ = 'ИВАН')	
  ORDER BY NSYST