Вот ваш запрос. использование union all
приведет к подсчету как назначенных, так и полных дат
select user_id
, complete_date as [assign_date / compete_date]
, sum(case when t1.s = 'assigned' then 1 else 0 end) as assign_task
, sum(case when t1.s = 'complete' then 1 else 0 end) as complete_task
from
(select user_Id, complete_date, 'complete' as s from task
union all
select user_Id, assign_date, 'assigned' as s from task) t1
group by t1.complete_date, t1.user_id