MySQL - Group By Contigous Blocks - mysql

Question

I am struggling to make a GROUP BY contiguous blocks, I've used the following two for references:
- GROUP BY for continuous rows in SQL
- How can I do a contiguous group by in MySQL?
- https://gcbenison.wordpress.com/2011/09/26/queries-that-group-tables-by-contiguous-blocks/
The primary idea that I am trying to encapsulate periods with a start and end date of a given state. A complexity unlike other examples is that I'm using a date per room_id as the indexing field (rather than a sequential id).
My table:
room_id | calendar_date | state
Sample data:
1 | 2016-03-01 | 'a'
1 | 2016-03-02 | 'a'
1 | 2016-03-03 | 'a'
1 | 2016-03-04 | 'b'
1 | 2016-03-05 | 'b'
1 | 2016-03-06 | 'c'
1 | 2016-03-07 | 'c'
1 | 2016-03-08 | 'c'
1 | 2016-03-09 | 'c'
2 | 2016-04-01 | 'b'
2 | 2016-04-02 | 'a'
2 | 2016-04-03 | 'a'
2 | 2016-04-04 | 'a'
The objective:
room_id | date_start | date_end | state
1 | 2016-03-01 | 2016-03-03 | a
1 | 2016-03-04 | 2016-03-05 | b
1 | 2016-03-06 | 2016-03-09 | c
2 | 2016-04-01 | 2016-04-01 | b
2 | 2016-04-02 | 2016-04-04 | c
The two attempts I've made at this:
1)
SELECT
rooms.row_new,
rooms.state_new,
MIN(rooms.room_id) AS room_id,
MIN(rooms.state) AS state,
MIN(rooms.date) AS date_start,
MAX(rooms.date) AS date_end,
FROM
(
SELECT #r := #r + (#state != state) AS row_new,
#state := state AS state_new,
rooms.*
FROM (
SELECT #r := 0,
#state := ''
) AS vars,
rooms_vw
ORDER BY room_id, date
) AS rooms
WHERE room_id = 1
GROUP BY row_new
ORDER BY room_id, date
;
This is very close to working, but when I print out row_new it starts to jump (1, 2, 3, 5, 7, ...)
2)
SELECT
MIN(rooms_final.calendar_date) AS date_start,
MAX(rooms_final.calendar_date) AS date_end,
rooms_final.state,
rooms_final.room_id,
COUNT(*)
FROM (SELECT
rooms.date,
rooms.state,
rooms.room_id,
CASE
WHEN rooms_merge.state IS NULL OR rooms_merge.state != rooms.state THEN
#rownum := #rownum+1
ELSE
#rownum
END AS row_num
FROM rooms
JOIN (SELECT #rownum := 0) AS row
LEFT JOIN (SELECT rooms.date + INTERVAL 1 DAY AS date,
rooms.state,
rooms.room_id
FROM rooms) AS rooms_merge ON rooms_merge.calendar_date = rooms.calendar_date AND rooms_merge.room_id = rooms.room_id
ORDER BY rooms.room_id, rooms.calendar_date
) AS rooms_final
GROUP BY rooms_final.state, rooms_final.row_num
ORDER BY room_id, calendar_date;
For some reason this is returning some null room_id's results as well as generally inaccurate.

Answer 1

Working with variables is a bit tricky. I would go for:
SELECT r.state_new, MIN(r.room_id) AS room_id, MIN(r.state) AS state,
MIN(r.date) AS date_start, MAX(r.date) AS date_end
FROM (SELECT r.*,
(#grp := if(#rs = concat_ws(':', room, state), #grp,
if(#rs := concat_ws(':', room, state), #grp + 1, #grp + 1)
)
) as grp
FROM (SELECT r.* FROM rooms_vw r ORDER BY ORDER BY room_id, date
) r CROSS JOIN
(SELECT #grp := 0, #rs := '') AS params
) AS rooms
WHERE room_id = 1
GROUP BY room_id, grp
ORDER BY room_id, date;
Notes:
Assigning a variable in one expression and using it in another is unsafe. MySQL does not guarantee the order of evaluation of expressions.
In more recent versions of MySQL, you need to do the ORDER BY in a subquery.
In the most recent versions, you can use row_number(), greatly simplifying the calculation.

Answer 2

Thanks to #Gordon Linoff for giving me insights to get to this answer:
SELECT
MIN(room_id) AS room_id,
MIN(state) AS state,
MIN(date) AS date_start,
MAX(date) AS date_end
FROM
(
SELECT
#r := #r + IF(#state <> state OR #room_id <> room_id, 1, 0) AS row_new,
#state := state AS state_new,
#room_id := room_id AS room_id_new,
tmp_rooms.*
FROM (
SELECT #r := 0,
#room_id := 0,
#state := ''
) AS vars,
(SELECT * FROM rooms WHERE room_id IS NOT NULL ORDER BY room_id, date) tmp_rooms
) AS rooms
GROUP BY row_new
order by room_id, date
;