MySQL GROUP BY DateTime +/- 3 seconds

Question

Suppose I have a table with 3 columns:

• id (PK, int)
• timestamp (datetime)
• title (text)

I have the following records:

Answer 1

Warning: Long answer. This should work, and is fairly neat, except for one step in the middle where you have to be willing to run an INSERT statement over and over until it doesn't do anything since we can't do recursive CTE things in MySQL.

I'm going to use this data as the example instead of yours:

id    Timestamp
1     1:00:00
2     1:00:03
3     1:00:06
4     1:00:10

Here is the first query to write:

SELECT a.id as aid, b.id as bid
FROM Table a
JOIN Table b 
ON (a.Timestamp is within 3 seconds of b.Timestamp)

It returns:

aid     bid
1       1
1       2
2       1
2       2
2       3
3       2
3       3
4       4

Let's create a nice table to hold those things that won't allow duplicates:

CREATE TABLE
Adjacency
( aid INT(11)
, bid INT(11)
, PRIMARY KEY (aid, bid) --important for later
)

Now the challenge is to find something like the transitive closure of that relation.

To do so, let's find the next level of links. by that I mean, since we have 1 2 and 2 3 in the Adjacency table, we should add 1 3:

INSERT IGNORE INTO Adjacency(aid,bid)
SELECT adj1.aid, adj2.bid
FROM Adjacency adj1
JOIN Adjacency adj2
ON (adj1.bid = adj2.aid)

This is the non-elegant part: You'll need to run the above INSERT statement over and over until it doesn't add any rows to the table. I don't know if there is a neat way to do that.

Once this is over, you will have a transitively-closed relation like this:

aid     bid
1       1
1       2
1       3     --added
2       1
2       2
2       3
3       1     --added
3       2
3       3
4       4

And now for the punchline:

SELECT aid, GROUP_CONCAT( bid ) AS Neighbors
FROM Adjacency
GROUP BY aid

returns:

aid     Neighbors
1       1,2,3
2       1,2,3
3       1,2,3
4       4

So

SELECT DISTINCT Neighbors
FROM (
     SELECT aid, GROUP_CONCAT( bid ) AS Neighbors
     FROM Adjacency
     GROUP BY aid
     ) Groupings

returns

Neighbors
1,2,3
4

Whew!