Aggregate tables setup

[CrPbI3yH_donuts]

Hello, 

I’m new one with looker and i’m trying to understand best practice for my task.

1. I have “actions” table that have rows for each action of user in unique “session”. I defined the view, created the explore for it.
   
   Now I need to calculate average number of actions in session. how can I do that? 
   
   I have tried to define aggregate table with “
   query {

    dimensions: [session_id]

    measures: [count]

}
   
   , but it uses sum function (when i try to use it in explore).
    
2. Also, I’ll need to evaluate the change of number of actions in percents for user comparing with previous session. Is it possible to do such calculations without definition of new objects (tables, views, pdt)? just by adding metrics.