Q&A for historians and history buffs
SELECT SUM(EndOfWeekUsersCount)+SUM(BeginOfWeekUsersCount) FROM --п.2 начало ( SELECT COUNT(DISTINCT OwnerUserId) AS BeginOfWeekUsersCount FROM Posts WHERE CreationDate BETWEEN DATEADD(day, 0, '2008-08-01') AND DATEADD(day, 1, '2008-08-01') ) BeginOfWeek --п.2 конец FULL JOIN --п.4 начало ( SELECT COUNT(begin_of_week.OwnerUserId) AS EndOfWeekUsersCount FROM ( SELECT DISTINCT OwnerUserId AS OwnerUserId FROM Posts WHERE CreationDate BETWEEN DATEADD(day, 7, '2008-08-01') AND DATEADD(day, 8, '2008-08-01') ) end_of_week INNER JOIN ( SELECT DISTINCT OwnerUserId AS OwnerUserId FROM Posts WHERE CreationDate BETWEEN DATEADD(day, 0, '2008-08-01') AND DATEADD(day, 1, '2008-08-01') ) begin_of_week ON end_of_week.OwnerUserId = begin_of_week.OwnerUserId ) EndOfWeek --п.4 конец ON BeginOfWeek.BeginOfWeekUsersCount = EndOfWeek.EndOfWeekUsersCount