不存在返回意外结果

Question

I am running a query to select approver credentials based on location, building number, etc. I want to have a default approver for a specific location, so an entry is made in the table with a Site value and NULL for Building and AcctDept. The result set is expected to be a single row.

Here is the query:

select distinct CS, Primary, Backup1, Backup2 
from Approvals 
where AcctDept = #ApprovalInformation.Account# 
            and Building = #ApprovalInformation.Building# 
            and Site = #ApprovalInformation.Site# 
            and AcctDept is not null 
            and Building is not null 
union 
select distinct CS, Primary, Backup1, Backup2 
from Approvals 
where AcctDept = #ApprovalInformation.Account# 
            and Site = #ApprovalInformation.Site# 
            and AcctDept is not null 
            and Building is null 

union select distinct CS, Primary, Backup1, Backup2 
from Approvals 
where Building = #ApprovalInformation.Building# 
            and Site = #ApprovalInformation.Site# 
            and AcctDept is null 
            and Building is not null 

union 
select distinct CS, Primary, Backup1, Backup2 
from Approvals 
where Site = #ApprovalInformation.Site# 
            and AcctDept is null 
            and Building is null 
and NOT EXISTS (
                    select * 
                    from Approvals 
                    where AcctDept = #ApprovalInformation.Account# 
                            and Building = #ApprovalInformation.Building# 
                            and Site = #ApprovalInformation.Site# 
                            and AcctDept is not null 
                            and Building is not null)

My thought was that if the first SELECT returns a value, then the default is not needed, and the NOT EXISTS statement will evaluate false. (The first select is just copy/pasted into the NOT EXISTS of the last statement)

However, when I run this, it is always evaluating to TRUE.

There must be something about the behavior of NOT EXISTS that I am missing?

Answer 1

select top 1 CS, Primary, Backup1, Backup2 
from Approvals 
where (AcctDept = #ApprovalInformation.Account# or AcctDept is null)
            and (Building = #ApprovalInformation.Building# or Building is null)
            and Site = #ApprovalInformation.Site# 
order by AcctDept, Building 

default ordering in oracle is asc, nulls last. With the top 1 clause this will give you one record back with non-null AcctDept and Building if available. Untested as no SQLFiddle was provided.