Avoid repeating same logic in 'select' and 'where' clauses?

Question

I'll preface by saying I'm self-taught and have only been fiddling with SQL for a couple of months, so forgive me if this is a dumb question. I have a query written to pull out customers who are configured to have their products stored at the wrong warehouse, according to the first 3 digits of the zip code. Here is an extremely simplified version of a query I'm trying to run:
select custno, custbuy_zip_cd, custbuy_prim_ship_loc_cd as Warehouse,
case when substr(custbuy_zip_cd,1,3) in ('839','848') then '20'
when substr(custbuy_zip_cd,1,3) in ('590','591') then '33'
end as StdWhse
from customers
where case when substr(custbuy_zip_cd,1,3) in ('839','848') then '20'
when substr(custbuy_zip_cd,1,3) in ('590','591') then '33'
end <> custbuy_prim_ship_loc_cd
or (case when substr(custbuy_zip_cd,1,3) in ('839','848') then '20'
when substr(custbuy_zip_cd,1,3) in ('590','591') then '33'
end is not null and custbuy_prim_ship_loc_cd is null)
Now, the query works, but it seems overly convoluted and feels like there must be a way to make it simpler and faster. I'm using the same 'case when' 3 times. Originally, I had thought I could use the aliases from the 'select' clause in the 'where' clause, which would simplify things:
select custno, custbuy_prim_ship_loc_cd as Warehouse,
case when substr(custbuy_zip_cd,1,3) in ('839','848') then '20'
when substr(custbuy_zip_cd,1,3) in ('590','591') then '33'
end as StdWhse
from customers
where StdWhse <> custbuy_prim_ship_loc_cd
or (StdWhse is not null and custbuy_prim_ship_loc_cd is null)
I then found out that that caused 'invalid identifier' errors. My first attempt at a solution was to use a subquery in the 'from' clause, but that ran the 'case when' on every single customer instead of the small subset, so it wound up taking much longer even though it looked neater. Any tips on how to clean up that first query to make it run faster?
this is Oracle 11i, I believe. As a side note, I don't have write access to the database.

Answer 1

Thanks for all the tips so far - still going through them. You all respond fast! Sorry about using double angle brackets for != and not using code tags, I'll make sure to format my posts properly going forward. I think the double angle brackets messed up the appearance of my original queries a little. Here's how I probably should have pasted my first query in my first post:
select custno, custbuy_zip_cd, custbuy_prim_ship_loc_cd as Warehouse,
    case when substr(custbuy_zip_cd,1,3) in ('839','848') then '20'
        when substr(custbuy_zip_cd,1,3) in ('590','591') then '33'
    end as StdWhse
from customers
where case when substr(custbuy_zip_cd,1,3) in ('839','848') then '20'
        when substr(custbuy_zip_cd,1,3) in ('590','591') then '33'
    end != custbuy_prim_ship_loc_cd
    or (case when substr(custbuy_zip_cd,1,3) in ('839','848') then '20'
        when substr(custbuy_zip_cd,1,3) in ('590','591') then '33'
    end is not null and custbuy_prim_ship_loc_cd is null)The almost unanimous opinion seems to be that I should use a subquery in one way or another, but the problem remains that the only significant logic to narrow down the results is the logic that matches the 'case when' results (which are what the warehouse number should be, based on the zip code) to the current warehouse number. Therefore, it seems like any subquery is going to return my entire list of 600k customers, and take a much longer time than my original (messy) query. At least it has in the test runs I created based on
Satyaki's and Peter's examples. The query based on my original example takes about 2.5 minutes, and the subquery examples take about 5+ even though they look cleaner.
to clarify what the query is trying to accomplish, I want it to pull any records where the warehouse number does not equal the correct warehouse number based on zip code (or if the warehouse number is null when it shouldn't be).
I'll try to create some sample data and sample results. Customers table:
custno   custbuy_zip_cd  custbuy_prim_ship_loc_cd
1        59024           20
2        59024           33desired results:
custno   custbuy_zip_cd   warehouse   StdWhse
1        59024            20          33If I could create a table to hold the standard warehouses to join on, the whole thing would be much easier. The full version of the query really has hundreds of zip code prefixes and 5 different warehouses and each account has 4 alternate warehouses as well. However, I'm stuck with read only access so everything has to go right in the query. It wouldn't be the end of the world to just stick with my original query since it's not like it takes hours, and I'll only be running it weekly. I just wanted to make sure there wasn't some other solution that wasn't just cleaner but was also faster.