Declare @Query varchar(1000)
Set @Query = 'Select ColumnName1, columnName2, ..... From TableName1 Where ColumnName1 = ''Test'' and '
IF isnull(@parm, '') = ''
Set @Query = @Query + 'CoulmnName2 in (Select CoulmnName2 from TableName2)'
ELSE
Set @Query = @Query + 'CoulmnName2 = ' + @parm
Execute @Query