SELECT <columns>
FROM   <table>
WHERE  db_column1 = NVL(:controlblock.field1, db_column1)
AND    db_column2 = NVL(:controlblock.field2, db_column2);

DECLARE
   CURSOR mysearch (criterium1 IN VARCHAR2, criterium2 IN VARCHAR2)
   IS
      WITH mytable AS
           (SELECT 'Jim' fname, 'Boston' locat
            FROM   DUAL
            UNION ALL
            SELECT 'Jenny' fname, 'Tucson' locat
            FROM   DUAL
            UNION ALL
            SELECT 'Frank' fname, 'Bristol' locat
            FROM   DUAL
            UNION ALL
            SELECT 'Patrick' fname, 'Boston' locat
            FROM   DUAL
            UNION ALL
            SELECT 'Susan' fname, 'Houston' locat
            FROM   DUAL)
      SELECT   *
      FROM     mytable
      WHERE    fname = NVL (criterium1, fname)
      AND      locat = NVL (criterium2, locat)
      ORDER BY fname;
BEGIN
   DBMS_OUTPUT.put_line ('show all: ');

   FOR rec1 IN mysearch (NULL, NULL)
   LOOP
      DBMS_OUTPUT.put_line (rec1.fname || ' is located in ' || rec1.locat);
   END LOOP;

   DBMS_OUTPUT.put_line ('show Boston only: ');

   FOR rec2 IN mysearch (NULL, 'Boston')
   LOOP
      DBMS_OUTPUT.put_line (rec2.fname || ' is located in ' || rec2.locat);
   END LOOP;

   DBMS_OUTPUT.put_line ('show Susan:');

   FOR rec3 IN mysearch ('Susan', NULL)
   LOOP
      DBMS_OUTPUT.put_line (rec3.fname || ' is located in ' || rec3.locat);
   END LOOP;
END;