Oracle SQL- Flag records based on record's date vs history

Question

This is my 1st post on the forum. Usually I was able to find what I needed - but to tell the truth - I am not really sure how to ask a correct question to the issue. Therefore,

Answer 1

Using a simplified "input" table... You can use the LAG() analytic function and a comparison condition to populate your last column. I assume your fiscalyearmonth is a number - if it is a character field, wrap fiscalyearmonth within TO_NUMBER(). (It would be much better if in fact you stored these as true Oracle dates, perhaps date 2016-06-01 instead of 201606, but I worked with what you have currently... and took advantage that in numeric format, "24 months ago" simply means "subtract 200").

with inputs (linenum, idnum, fiscalyearmonth) as (
       select 1, 255, 201605 from dual union all
       select 2, 123, 201602 from dual union all
       select 3, 255, 201601 from dual union all
       select 4, 255, 201210 from dual
     )
select linenum, idnum, fiscalyearmonth, 
       case when fiscalyearmonth 
                 - lag(fiscalyearmonth) 
                           over (partition by idnum order by fiscalyearmonth) < 200
            then 'Existing' else 'New' end         as flag
from   inputs
order by linenum;


   LINENUM      IDNUM FISCALYEARMONTH FLAG
---------- ---------- --------------- --------
         1        255          201605 Existing
         2        123          201602 New
         3        255          201601 New
         4        255          201210 New