Sql query selct statement

Hi,

I have a table with customer number and memo types left on their account.
e.g.

---------------------------
Customermemo
---------------------------
cnum     memotype
a1               debt
b1                good
a1                paid
b1                debt
b3                new
b2                leaving
b2                left
b3                paid

I have another table with only customer number
----------------
Customer
------------------
customerno
a1
b1
a2
b2
b3
a3

Now i am trying to get distinct customer number without memotype debt ( that customer number should be in both tables)

here is a query

select distinct(cnum) from customermemo, customer where
memotype <> 'DEBT'
and
cnum = customerno

But still i kept on getting a1 in the output though i do not want anything with the memo type DEBT.