Declare @selector nvarchar(2)
Set @selector = 'A';
Select ab,bc,cd from day where 1=case when @selector = 'A' then 1 else 0 end
Union all
Select ab,bc,cd from month where 1=case when @selector = 'B' then 1 else 0 end
Union all
Select ab,bc,cd from year where 1=case when @selector = 'C' then 1 else 0 end
)