SQL & PL/SQL

Hi,
I have a weird problem with SQL parameter passing. I will be grateful if someone can help me out.
Database is Oracle 11G. If I run:
select 1 from msf952
where dstrct_code = upper('CTH')
and parent_acct = upper('18ITDPT')

26 rows will be returned.

However if I run:

VAR StartAccount Varchar2(100);
exec :StartAccount :='18ITDPT';
VAR District VARCHAR2(100);
exec :District:='CTH';

select 1 from msf952
where dstrct_code = :District
and parent_acct = :StartAccount

I have 0 row returned.

I've tested
VAR StartAccount Varchar2(100);
exec :StartAccount :='18ITDPT';
VAR District VARCHAR2(100);
exec :District:='CTH';

select :StartAccount, :District from dual;
It worked so it means values have been assigned to my parameters and can be referenced by query script.

So can anyone tell me why my query can't return proper data when I put parameters in the where clause?
Thanks a lot.




select 1 from msf952
where dstrct_code = upper('CTH')
and parent_acct = upper('18ITDPT')