Do you have the value NULL in column client_accountno? It seems that you have it. You have to take into account NULLs. Check the code bellow that shows it:

--Creating the tables

create table Demo (I INT NULL)

go

create table Demo2 (I INT NULL)

go

--Inserting the data

insert into Demo (I)

select 1 union select 2

go

insert into Demo2 (I)

select 1 union select 3

go

--At this point Demo2 does not

--contain the value NULL, so this should work

select * from Demo

where I not in (select I from Demo2)

select * from Demo

where not exists (select Demo2.I from Demo2 where Demo2.I = Demo.I)

--After inserting the value Null the first query

--won't return any records

insert Demo2 (I) values (NULL)

select * from Demo

where I not in (select I from Demo2)

--I need to exclude the value Null from the subquery

select * from Demo

where not exists (select Demo2.I from Demo2 where Demo2.I = Demo.I)

--cleanup

drop table Demo

drop table Demo2

Adi