Oracle selects data using adjacent date blocks

Question

I am using an Oracle database and I have a table with two columns with this data:

HASH | DATE
-----------------
abcd | 2017-11-01
abcd | 2017-11-02
abcd | 2017-11-03
wxyz | 2017-11-04
wxyz | 2017-11-05
abcd | 2017-11-06
wxyz | 2017-11-07
abcd | 2017-11-08
abcd | 2017-11-09
lmno | 2017-11-10
lmno | 2017-11-11

I want to know the time windows that every hash is visible. So that,

hash | start      | end
------------------------------
abcd | 2017-11-01 | 2017-11-03
wxyz | 2017-11-04 | 2017-11-05
abcd | 2017-11-06 | 2017-11-06
wxyz | 2017-11-07 | 2017-11-07
abcd | 2017-11-08 | 2017-11-09
lmno | 2017-11-10 | 2017-11-11

What I still have is basically this:

SELECT HASH, MIN(DATE) ST, MAX(DATE) ED
FROM HASH_TABLE 
GROUP BY HASH 
ORDER BY 3 DESC

And it almost works, but it will give me both “abcd” at the beginning of 2017-11-01 and the end of 2017-11-09, which “hides” the fact that it switched in the middle.

Is there a way to group these results using adjacent "block" dates / times?

Answer 1

It seems the problem of "spaces and islands":

WITH cte("hash","date") AS (
    SELECT 'abcd',  DATE'2017-11-01' FROM dual UNION ALL
    SELECT 'abcd',  DATE'2017-11-02' FROM dual UNION ALL
    SELECT 'abcd',  DATE'2017-11-03' FROM dual UNION ALL
    SELECT 'wxyz',  DATE'2017-11-04' FROM dual UNION ALL
    SELECT 'wxyz',  DATE'2017-11-05' FROM dual UNION ALL
    SELECT 'abcd',  DATE'2017-11-06' FROM dual UNION ALL
    SELECT 'wxyz',  DATE'2017-11-07' FROM dual UNION ALL
    SELECT 'abcd',  DATE'2017-11-08' FROM dual UNION ALL
    SELECT 'abcd',  DATE'2017-11-09' FROM dual UNION ALL
    SELECT 'lmno',  DATE'2017-11-10' FROM dual UNION ALL
    SELECT 'lmno',  DATE'2017-11-11' FROM dual 
)
select "hash"
      ,min("date") as startdate
      ,max("date") as enddate
from (
    select "date","hash"
         , row_number() over (order by "date") 
         - row_number() over (partition by "hash" order by "date") as grp
    from cte
) A
group by "hash", grp
ORDER BY startdate;

DBFiddle Demo

Answer 2

desc hash_table

hash varchar2(4)
 date_ date

select hash, min(date_) start_, max(date_) end_
  from
    (
     select h.hash, h.date_, row_number() over (partition by hash order by date_) rn
       from hash_table h 
    )
  group by hash, date_ - rn
  order by 2;