获取每个组中元素之间的最小差异

Question

I have a mysql table in the following form:

• timestamp (unix timestamp)
• some value columns [...]
• groupid

Now I want to build a query which groups by groupid and prints the minimum time difference between each of the timestamps in the group.

Something like SELECT groupid, MIN(??? - ???) as minimum_time_diff FROM mytable GROUP BY groupid;

Any help would be appreciated!

Answer 1

One way is with a self-join and group by :

select t1.groupid, min(timestampdiff(second, t1.timestamp, t2.timestamp)) as minsecs
from table t1 join
     table t2
     on t1.groupid = t2.groupid and t1.timestamp < t2.timestamp
group by t1.groupid;

You can also do this with variables:

select groupid, min(timestampdiff(second, prevtimestamp, timestamp))
from (select t.*,
             if(@groupid = groupid, @prevtimestamp, NULL) as prevtimestamp,
             @prevtimestamp := timestamp,
             @groupid = groupid
      from table t cross join
           (select @prevtimestamp := 0, @groupid = '')
      order by groupid, timestamp
     ) t
group by groupid;

This uses the observation that when the timestamps are ordered, then the minimum difference is going to occur between two adjacent rows.