Find the maximum consecutive years for each ID's in a table(Oracle SQL)

Question

I am trying to solve a problem of how to find the maximum count of consecutive years in a series of records. In the following example:

ID  Year
1 1993
1 1994
1 19         


        

Answer 1

Try:

with cte as
(select t.id, t.year, d.d, row_number() over (partition by t.id, d.d 
                                              order by t.year) rn
 from (select -1 d from dual union all select 1 d from dual) d
 cross join my_table t 
 where not exists
       (select null
        from my_table o
        where t.id = o.id and t.year = o.year-d.d) )
select s.id, max(e.year-s.year)+1 year_count
from cte s
join cte e on s.id = e.id and s.rn = e.rn and e.d=1
where s.d=-1
group by s.id

SQLFiddle here.

Answer 2

This will produce your desired result:

select
  id,
  ayear,
  byear,
  yeardiff
from
(
  select
    a.id,
    a.year ayear,
    b.year byear,
    (b.year - a.year)+1 yeardiff,
    dense_rank() over (partition by a.id order by (b.year - a.year) desc) rank
  from
    years a
    join years b on a.id = b.id 
        and b.year > a.year
  where
    b.year - a.year = 
      (select count(*)-1
         from years a1
        where a.id = a1.id
             and a1.year between a.year and b.year)
)
where
  rank = 1

EDIT updated to display start/end years of longest stretch.

SQLFiddle