If Else logic converting to Select case - Oracle - oracle

Question

IF inputVariable = '0'
THEN
DELETE existingTemptable
WHERE status != 999;
ELSE
IF inputVariable = '1'
THEN
DELETE existingTemptable
WHERE status = 999;
ELSE
DELETE existingTemptable
WHERE status != 999
AND date < utils.dateadd('MONTH', -6, SYSTIMESTAMP);
END IF;
END IF;
This If logic is present on a temp table and I have to remove the temp table and make it only select query , so approached with WITH CTE but stuck in the below
what should be in the where clause
with existingTemptable as
(
//got the temp table here
), myTable as
(
Select * from existingTemptable
**where
status = CASE WHEN inputVariable = '0' THEN 999
WHEN inputVariable != '0' AND inputVariable != '1' THEN 999
ELSE status
END**
)Select * from myTable
What to put in WHERE clause so that it mimics the If logic above

Answer 1

You have to be careful if inputVariable or status (or both) may be null. The rest is a direct application of logical rules one learns (or should learn) in middle school.
select *
from existingTemptable
where inputVariable = '0' and (status = 999 or status is null)
or
inputVariable = '1' and (status != 999 or status is null)
or
(inputVariable is null or inputVariable not in ('0', '1'))
and (status = 999 or status is null) and date >= [whatever]
Note - even in the absence of null handling, you can't write the where clause with a case expression; that's because in one branch you require "not equal", and you can't do it with a simple case expression.