MySQL - Find the largest time difference between consecutive datetimes

This query will compute time differences between successive rows and display the largest one -- the length of the winning streak. If you need the whole row, you'll need Gordon's query.

SELECT MAX(diff)
  FROM (
          SELECT TIMEDIFF(datetime,@prev) AS diff,
                 (@prev:=datetime) AS datetime  
            FROM resets, 
                 (SELECT @prev:=(SELECT MIN(datetime) FROM resets)) AS init
           ORDER BY datetime
       ) AS diffs

How does this work?

First of all, it's a cross-join between a one-row query and your table. The one-row query is this:

       (SELECT @prev:=(SELECT MIN(datetime) FROM resets)) 

It sets the user-defined value @prev to the lowest/earliest datetime in the table. This is a MySQL trick to initialize a user-defined variable at the beginning of the query.

Then, the SELECT clause has two columns in it:

          SELECT TIMEDIFF(datetime,@prev) AS diff,
                 (@prev:=datetime) AS datetime  

The first one takes the time difference between the present row's datetime and the value of @prev. The second one updates the value of @prev to the present row's datetime.

So the inner query spits out a list of the timestamps and the difference to the preceding timestamp in ORDER BY datetime.

The outer query SELECT MAX(diff) grabs the largest value of diff -- the longest winning streak -- from the inner query.

Let's be clear: This is MySQL-specific monkey business. Pure SQL is supposed to be declarative, not procedural. But this trick with the user-defined @prev variable lets us mix declarative and procedural code in a useful way, even if it's somewhat obscure.

You can calculate the next datetime using a correlated subquery and then find the biggest by sorting:

select r.*
from (select r.*,
             (select datetime
              from resets r2
              where r2.datetime > r.datetime
              order by datetime
              limit 1
             ) as next_datetime
      from resets r
     ) r
order by timestampdiff(second, datetime, next_datetime) desc
limit 1;