Create a query that fetches rows plus their adjacent rows - mysql

Question

I am having problems with a specific query - respectively creating the query in the first place.
The columns can be reduced to id, seconds and status.
=============================
| id | seconds | status |
-----------------------------
| 0 | 0 | 0 |
| 1 | 12 | 1 |
| 2 | 25 | 0 |
| 3 | 37 | 1 |
| 4 | 42 | 0 |
=============================
What I'd like to have: All entries with status = 1 PLUS all entries that are less than 10 seconds away from those entries. Basically, I want to fetch all possible pairs (or triplets, etc.) of rows to check manually (later automatically) whether they need to be paired (there is a column parent_id for this purpose, but we don't need that for the query). I could do this in code (first select all status=1, then loop), but I wonder whether it is possible to do this purely in the database.
Thus, my desired output would be the following:
=============================
| id | seconds | status |
-----------------------------
| 1 | 12 | 1 | <- status = 1
| 3 | 37 | 1 | <- status = 1
| 4 | 42 | 0 | <- only 5 seconds after status = 1
=============================
My current best guess is this:
SELECT * FROM entries e0
WHERE
e0.status = 1 OR
e0.status = 0 AND
0 < (SELECT count(*)
FROM entries e1
WHERE e1.status = 1 AND abs(e1.seconds - e0.seconds) < 10)
But this fetches the whole table, and I don't really know why - and it takes a long time to do so (there is an index on the column seconds, the table has 9000 entries).
Is there a way to do this (maybe even effiently)?

Answer 1

Here's one option with union all and exists:
select * from entries where status = 1
union all
select * from entries e where status = 0 and
exists (select 1
from entries e2
where e2.status = 1 and
abs(e.seconds - e2.seconds) < 10
)
SQL Fiddle Demo
Alternatively you could use an outer join with distinct instead of exists:
select distinct e.*
from entries e
left join entries e2 on e2.status = 1
where e.status = 1 or abs(e.seconds - e2.seconds) < 10
More Fiddle

Answer 2

I prefer to do it in a single query. However there are also ways of doing it with exists or subqueries as well. Utilizing an outer join means you can grab everything at once with a nicely crafted where and join statements, adding a group by or distinct based on your performance situation will tidy up your results and make them unique rows.
My suggestion on where statements to ensure your intentions are met is to use parenthesis to establish your intended precedence. It will make your code clearer to your intentions.
WHERE Condition1 = True OR Condition2 = True AND Condition3 = True
Should be
WHERE Condition1 = True OR (Condition2 = True AND Condition3 = True)
Oddly, I would not have thought it would evaluate in the manner you mention because of past experience but then again I ALWAYS use parenthesis to establish my precedence to make it more clear and easier to craft more complex conditions.
Reason you are getting the whole table. Is because of the data in your table. Seriously, sometimes we go looking for the answer and make it complicated, I prefer my way of solving the query of yours but given your result set example my query and yours get the same results! Try changing the 10 seconds down to 1/2/3 etc and see what the effect of your query is. My assumption would be in your full dataset that your any record with a status of 0 is within 10 seconds of a record that has a status of 1...... I would have commented back but this is one of the first questions I have answered.
Here is some example code based on your dataset and query.
DECLARE #Entries AS TABLE (
Id INT
,Seconds INT
,[Status] BIT
)
INSERT INTO #Entries (Id, Seconds, [Status])
VALUES (0,0,0 )
,(1,12,1 )
,(2,25,0 )
,(3,37,1 )
,(4,42,0 )
SELECT *
FROM
#Entries e0
WHERE
e0.Status = 1
OR e0.Status = 0
AND 0 < (SELECT count(*)
FROM
#Entries e1
WHERE e1.Status = 1 AND ABS(e1.Seconds - e0.Seconds) < 10)
SELECT DISTINCT
e0.*
FROM
#Entries e0
LEFT JOIN #Entries e1
ON e1.[Status] = 1
AND ABS(e1.seconds - e0.seconds) < 10
WHERE
e0.[Status] = 1
OR e1.id IS NOT NULL