How to find "holes" in a table

Question

I recently inherited a database on which one of the tables has the primary key composed of encoded values (Part1*1000 + Part2).
I normalized that column, but I cannot change the old values. So now I have

select ID from table order by ID
ID
100001
100002
101001
...

I want to find the "holes" in the table (more precisely, the first "hole" after 100000) for new rows.
I'm using the following select, but is there a better way to do that?

select /* top 1 */ ID+1 as newID from table
where ID > 100000 and
ID + 1 not in (select ID from table)
order by ID

newID
100003
101029
...

The database is Microsoft SQL Server 2000. I'm ok with using SQL extensions.

Answer 1

select ID +1 From Table t1
where not exists (select * from Table t2 where t1.id +1 = t2.id);

not sure if this version would be faster than the one you mentioned originally.

Answer 2

SELECT (ID+1) FROM table AS t1
LEFT JOIN table as t2
ON t1.ID+1 = t2.ID
WHERE t2.ID IS NULL