begin transaction
update ...
if @@rocount = 0
insert...
commit transaction

if(exists(select * from T with(rowlock, SERIALIZABLE ) where id = @id))

update T with(rowlock, SERIALIZABLE )
...

create table rrr1(id int, data varchar(10));
go

create unique clustered index uidx_rrr1 on rrr1(id);
go

set showplan_text on;
go

begin transaction;

merge
	into rrr1 as trgt
	using (select 1, '12345') as src(id, data)
	on
		trgt.id = src.id
	when matched then
		update
		set
			trgt.data = src.data
	when not matched then
		insert(id, data)
		values(src.id, src.data);

rollback transaction;
go

set showplan_text off;
go

  |--Clustered Index Merge(OBJECT:([db1].[dbo].[rrr1].[uidx_rrr1] AS [trgt]), SET:(Insert, [db1].[dbo].[rrr1].[data] as [trgt].[data] = [Expr1007],[db1].[dbo].[rrr1].[id] as [trgt].[id] = [Expr1008]), SET:(Update, [db1].[dbo].[rrr1].[data] as [trgt].[data] = [Expr1007]) ACTION:([Action1006]))
       |--Compute Scalar(DEFINE:([Expr1007]=CONVERT_IMPLICIT(varchar(10),[Expr1001],0), [Expr1008]=CASE WHEN [Action1006]=(4) THEN [Expr1000] ELSE [db1].[dbo].[rrr1].[id] as [trgt].[id] END))
            |--Compute Scalar(DEFINE:([Action1006]=ForceOrder(CASE WHEN [TrgPrb1004] IS NOT NULL THEN (1) ELSE (4) END)))
                 |--Compute Scalar(DEFINE:([Expr1000]=(1), [Expr1001]='12345'))
                      |--Nested Loops(Left Outer Join)
                           |--Constant Scan
                           |--Compute Scalar(DEFINE:([TrgPrb1004]=(1)))
                                |--Clustered Index Seek(OBJECT:([db1].[dbo].[rrr1].[uidx_rrr1] AS [trgt]), SEEK:([trgt].[id]=(1)) ORDERED FORWARD)

insert t(id,d)
select @p_id, @p_d
where not exists(select 1 from t where id = @p_id)
update t set d=@p_d
where id = @p_id