time | total | mice_ids (a json string. or use mongo)
And then, run through all your data and assign each mouse to a certain duration (ie, minute one, minute two, etc), updating the total counts every time as well as the mice_ids. Then, you could query by minute and see which minutes had meetings in them (ie, select mice where total > 2)
There are four types of meeting: mouse 1 entered -> mouse 2 entered -> mouse 2 left -> mouse 1 left; mouse 1 entered -> mouse 2 entered -> mouse 1 left -> mouse 2 left; mouse 2 entered -> mouse 1 entered -> mouse 1 left -> mouse 2 left; mouse 2 entered -> mouse 1 entered -> mouse 2 left -> mouse 1 left.
Then the meeting record contains the times the two were together and the duration of this.
The dumb approach is very slow. Would it be a good idea to read all stay results in Scala (or whatever) and try to do the searching in memory, completely eliminating th db? But then the comparisons would be more expensive...
It's not like you're ingesting gobs and gobs of data in real time :)