forespørgsel for at ignorere duplikerede/nul-poster

brug venstre join til tabel C

  select a.*,b.*, c.DeclareDate, c.Value from TableA a
   inner join TableB b on  a.AI =b.BId
  left join 
  (select * from TableC where DeclareDate='December') c
   on b.BId = c.CId

OUTPUT