MySQL GROUP BY DateTime +/- 3 seconds

I'm using Tom H.'s excellent idea but doing it a little differently here:

Instead of finding all the rows that are the beginnings of chains, we can find all times that are the beginnings of chains, then go back and ifnd the rows that match the times.

Query #1 here should tell you which times are the beginnings of chains by finding which times do not have any times below them but within 3 seconds:

SELECT DISTINCT Timestamp
FROM Table a
LEFT JOIN Table b
ON (b.Timestamp >= a.TimeStamp - INTERVAL 3 SECONDS
    AND b.Timestamp < a.Timestamp)
WHERE b.Timestamp IS NULL

And then for each row, we can find the largest chain-starting timestamp that is less than our timestamp with Query #2:

SELECT Table.id, MAX(StartOfChains.TimeStamp) AS ChainStartTime
FROM Table
JOIN ([query #1]) StartofChains
ON Table.Timestamp >= StartOfChains.TimeStamp
GROUP BY Table.id

Once we have that, we can GROUP BY it as you wanted.

SELECT COUNT(*) --or whatever
FROM Table
JOIN ([query #2]) GroupingQuery
ON Table.id = GroupingQuery.id
GROUP BY GroupingQuery.ChainStartTime

I'm not entirely sure this is distinct enough from Tom H's answer to be posted separately, but it sounded like you were having trouble with implementation, and I was thinking about it, so I thought I'd post again. Good luck!

Now that I think that I understand your problem, based on your comment response to OMG Ponies, I think that I have a set-based solution. The idea is to first find the start of any chains based on the title. The start of a chain is going to be defined as any row where there is no match within three seconds prior to that row:

SELECT
    MT1.my_id,
    MT1.title,
    MT1.my_time
FROM
    My_Table MT1
LEFT OUTER JOIN My_Table MT2 ON
    MT2.title = MT1.title AND
    (
        MT2.my_time < MT1.my_time OR
        (MT2.my_time = MT1.my_time AND MT2.my_id < MT1.my_id)
    ) AND
    MT2.my_time >= MT1.my_time - INTERVAL 3 SECONDS
WHERE
    MT2.my_id IS NULL

Now we can assume that any non-chain starters belong to the chain starter that appeared before them. Since MySQL doesn't support CTEs, you might want to throw the above results into a temporary table, as that would save you the multiple joins to the same subquery below.

SELECT
    SQ1.my_id,
    COUNT(*)  -- You didn't say what you were trying to calculate, just that you needed to group them
FROM
(
    SELECT
        MT1.my_id,
        MT1.title,
        MT1.my_time
    FROM
        My_Table MT1
    LEFT OUTER JOIN My_Table MT2 ON
        MT2.title = MT1.title AND
        (
            MT2.my_time < MT1.my_time OR
            (MT2.my_time = MT1.my_time AND MT2.my_id < MT1.my_id)
        ) AND
        MT2.my_time >= MT1.my_time - INTERVAL 3 SECONDS
    WHERE
        MT2.my_id IS NULL
) SQ1
INNER JOIN My_Table MT3 ON
    MT3.title = SQ1.title AND
    MT3.my_time >= SQ1.my_time
LEFT OUTER JOIN
(
    SELECT
        MT1.my_id,
        MT1.title,
        MT1.my_time
    FROM
        My_Table MT1
    LEFT OUTER JOIN My_Table MT2 ON
        MT2.title = MT1.title AND
        (
            MT2.my_time < MT1.my_time OR
            (MT2.my_time = MT1.my_time AND MT2.my_id < MT1.my_id)
        ) AND
        MT2.my_time >= MT1.my_time - INTERVAL 3 SECONDS
    WHERE
        MT2.my_id IS NULL
) SQ2 ON
    SQ2.title = SQ1.title AND
    SQ2.my_time > SQ1.my_time AND
    SQ2.my_time <= MT3.my_time
WHERE
    SQ2.my_id IS NULL

This would look much simpler if you could use CTEs or if you used a temporary table. Using the temporary table might also help performance.

Also, there will be issues with this if you can have timestamps that match exactly. If that's the case then you will need to tweak the query slightly to use a combination of the id and the timestamp to distinguish rows with matching timestamp values.

EDIT: Changed the queries to handle exact matches by timestamp.

MySQL GROUP BY DateTime +/- 3 seconds

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