a complicated calculation query.. for me

There is a database with the following structure:

var1: unique identifier
var2: eventtime
var3: activity

Each unique identifier can have multiple eventtimes because the unique identifier refers to individual bees in an experiment. Some unique identifiers have 3 eventtimes, others have 400.

The goal is to create a calculated field where each subsequent eventtime for a given unique identifier is compared to the first one. Let me show an example of what I mean:

16356 ....... 0
16356 ....... 5
16356 ....... 7
52656 ....... 0
52656 ....... 1
... etc

How would I create this calculated field?
Below is my (bad) attempt at getting this solution:

select uid, YEAR(eventtime), MONTH(eventtime), DAY(eventtime), HOUR(eventtime), MINUTE(eventtime), uid||eventtime in (select uid||time_to_sec(timediff(eventtime,min(eventtime))) as elapsed from alldata), activity
       from alldata
       where activity in ('concentric','radial')
       group by uid, YEAR(eventtime), MONTH(eventtime), DAY(eventtime), HOUR(eventtime), MINUTE(eventtime)
       order by uid, eventtime

Code (markup):

Thanks!