
select 
   udates.ts, count(*) 
from 
   taxdata_1,
   (
      select distinct ts from
      (
         select startdatetime as ts from taxdata_1
         union
         select enddatetime from taxdata_1
      ) as dates
   ) as udates
where 
   startdatetime <= udates.ts and
   enddatetime >= udates.ts
group by
   udates.ts
order by 
   udates.ts
;
