SQL> create table t(x int, y int);
SQL> commit;
SQL> insert into t values(1, 100);
SQL> insert into t values(2, 200);
SQL> insert into t values(3, 400);
SQL> commit;

SQL> update t set y=null where (select count(*) from t where y is null)=0;
SQL> select * from t;

           X            Y
============ ============
           2       <null>
           3       <null>
           1       <null>
-- OK, так и должно быть.
SQL> rollback;

SQL> set term ^;
SQL> create or alter procedure sp returns(cnt int) as
CON> begin
CON> select count(*) from t where y is null into cnt;
CON> suspend;
CON> end^
SQL> set term ;^
SQL> commit;
SQL> update t set y=null where (select cnt from sp)=0;
SQL> select * from t;

           X            Y
============ ============
           2       <null>
           3       <null>
           1       <null>
-- это тоже ОК
SQL> rollback;

SQL> set term ^;
SQL> recreate function fn returns int as
CON> begin
CON> return (select count(*) from t where y is null);
CON> end^
SQL> set term ;^

SQL> update t set y=null where fn()=0;
SQL> select * from t;

           X            Y
============ ============
           2       <null>
           3          400
           1          100
-- Это как это ?