Search data from sql database All data and with condition using single query

//Sql Query

declare @tt int
set @tt=0
select * from mstCombobox_T where (@tt<>0 and Flag=@tt) or (Flag<>0 and @tt=0)

//Output

declare @tt int
set @tt=12
select * from mstCombobox_T where (@tt<>0 and Flag=@tt) or (Flag<>0 and @tt=0)


//Output