MySQL select rows based on previous and next rows - mysql

Question

Suppose I have a table looking like:
mytable
category | begintime
---------|----------------------
cat1 | 2016-09-25 15:00:00
cat2 | 2016-09-25 16:00:00
cat1 | 2016-09-25 17:30:00
cat3 | 2016-09-25 19:00:00
cat1 | 2016-09-25 20:00:00
: :
Note that it doesn't have an ID-number, the begintime column is my primary key.
In the end, I would like to select all rows that are surrounded by a certain category, that is, to select all rows such that the category from the previous row is #catBefore and the category from the next row is #catAfter.
For example, what I would like is something like:
SELECT * FROM mytable WHERE previousRow.category = 'cat1' AND nextRow.category = 'cat1'
resulting in
SELECT * FROM ...
category | begintime
---------|----------------------
cat2 | 2016-09-25 16:00:00
cat3 | 2016-09-25 19:00:00
: :
The previousRow and nextRow in this, don't seem to be definable.
Idea
I have tried some things, but nothing has worked out yet. One of my ideas was to first select the previous and next category as new columns, so something like:
SELECT mytable.*,
previousRow.category AS prevCat,
nextRow.category AS nextCat
FROM mytable, [stuff-I-don't-know]
resulting in
SELECT mytable.* ...
category | begintime | prevCat | nextCat
---------|----------------------|---------|---------
cat1 | 2016-09-25 15:00:00 | null | cat2
cat2 | 2016-09-25 16:00:00 | cat1 | cat1
cat1 | 2016-09-25 17:30:00 | cat2 | cat3
cat3 | 2016-09-25 19:00:00 | cat1 | cat1
cat1 | 2016-09-25 20:00:00 | cat3 | ...
: :
and then filtering using a WHERE clause.
Is this idea possible, or could it be done in some other way?

Answer 1

One method uses correlated subqueries. This should be okay performance-wise, if you do indeed have primary key declarations:
select t.*
from (select t.*,
(select t2.category
from mytable t2
where t2.begintime < t.begintime
order by begintime desc
limit 1
) as prev_category,
(select t2.category
from mytable t2
where t2.begintime > t.begintime
order by begintime asc
limit 1
) as next_category
from mytable t
) t
where prev_category = #cat1 and next_category = #cat2;
EDIT:
You can do this with variables:
select t.*
from (select t.*,
(#pn := (case when (#pcy := #pn) = NULL then -1 -- never gets here
when (#pn := category) = NULL then -1 -- never gets here
else #pcy
end)
) as next_category
from (select t.*,
(#pc := (case when (#pcx := #pc) = NULL then -1 -- never gets here
when (#pc := category) = NULL then -1 -- never gets here
else #pcx
end)
) as prev_category
from t cross join
(select #pc := '') params
order by t.begintime
) t cross join
(select #pn := '') params
order by t.begintime desc
) t
where prev_category = #cat1 and next_category = #cat2;

Answer 2

I did this one... it seems to work, but it seems too slow actually... =\ I'll try to improve it. Please, check if it works for you:
SELECT
t1.category,
t1.begintime
FROM myTable t1 -- current
INNER JOIN myTable t2 ON 1=1 -- prev
AND t2.begintime < t1.begintime
INNER JOIN myTable t3 ON 1=1 -- next
AND t3.begintime > t1.begintime
LEFT JOIN myTable t4 ON 1=1 -- between current and prev
AND t4.begintime < t1.begintime
AND t4.begintime > t2.begintime
LEFT JOIN myTable t5 ON 1=1 -- between current and next
AND t5.begintime > t1.begintime
AND t5.begintime < t3.begintime
WHERE 1=1
AND t2.category = 'cat1' -- prev cat
AND t3.category = 'cat1' -- next cat
AND t4.begintime IS NULL -- nothing between current and prev
AND t5.begintime IS NULL -- nothing between current and next
;