select t.plan_source_value, t.pat_cnt as num_patients,
       100.00*t.pat_cnt/ (sum(t.pat_cnt) over()) perc_of_total_count  
from (
  select p.plan_source_value,
         count(1) as pat_cnt
  from payer_plan_period p
  group by p.plan_source_value
) t
order by t.plan_source_value
;