Dear all,
I need to perform a kind of partial GROUP BY to determine the beginnings and
ends of sets of identical data. I can't use a full GROUP BY because these sets
can be repeated and their repetition must be conserved. Other database engines
have solutions for this task (like windowing in postgre) but I wonder if there
is an efficient recipe in SQLite.
Example:
=======
Table: mytable
--------
date test
------ ------
1 clim
3 clim
7 amb
10 amb
13 clim
15 clim
20 clim
22 amb
25 amb
Desired result
---------------------
date_from date_to test
--------------- ------------ ------
1 3 clim
7 10 amb
13 15 clim
22 25 amb
(non optimal) solution found
=====================
CREATE VIEW mytablebydate
AS -- Pre-order table to avoid ordering it twice in sub-queries
SELECT * FROM mytable ORDER BY date
CREATE VIEW mytablenext
AS
SELECT date,
test,
(
-- first row > this row
SELECT date -- NULL if not exists
FROM mytablebydate
WHERE date > MT.date
LIMIT 1
) as date_next,
(
-- first row > this row
SELECT test -- NULL if not exists
FROM mytablebydate
WHERE date > MT.date
LIMIT 1
) as test_next
FROM mytable MT
WHERE test != test_next
-- Get desired results
SELECT (
-- Date of the previous row
SELECT MAX( date_next )
FROM mytablenext
WHERE date_next < mt.date
) AS date_from,
date AS date_to, -- this row
test
FROM mytablenext mt
Comments
========
This method returns a Null for the first date_from and the last group is not
returned. It is therefore incomplete. In addition, it involves quite a lot of
subqueries. For completeness, it is inspired by
http://stackoverflow.com/questions/30455227/date-range-for-set-of-same-data/30460263#30460263.
So, is there a better / official way in SQLite ?
Any help will be welcome,
Jonathan
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