MySQL solution
select\nround(sum(case when a.min +1 = a2.event_date then 1 else 0 end) / count(distinct a.player_id),2) as fraction\nfrom \n(select player_id, min(event_date) 
select\nround(sum(case when a.min +1 = a2.event_date then 1 else 0 end) / count(distinct a.player_id),2) as fraction\nfrom \n(select player_id, min(event_date) as min\nfrom activity\ngroup by 1) a join activity a2 on a.player_id = a2.player_id
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