SQL UPDATE Self-join with compound matching value - sql-server

Question

I have a MSSQL table that contains rows, some of which I need to update, depending upon the absence/presence of other rows in the same table. the table and sample data look like this:
SQL Table:
TABLE tblConnector(
ID1 [int] NOT NULL,
ID2 [int] NOT NULL,
ID3 [int] NOT NULL,
...other cols...)
having PRIMARY KEY CLUSTERED comprised of ID1, ID2 and ID3 - meaning that the compound key must be unique.
Data Structure:
ID1 ID2 ID3
111 222 1
111 222 9999
333 444 1 <--- update only this row
555 666 1
555 666 9999
777 888 2
777 888 9999
123 456 3 <--- don't update this row
I need to update any row in this table, setting ID3 = 9999, where ID3 currently is 1 and there are no other rows in the table with the same values for ID1 and ID2, with ID3 = 9999.
In my sample data, I only want to update the 3rd row, setting ID3 = 9999 - because it has ID3 = 1 and there are no other rows in the table with the same ID1 and ID2 values (where ID3 <> 1). I don't want to update the last row - so using count() to isolte rows isn't a valid approach.
I've tried numerous attempts to join the table to itself, but I can't seem to get an UPDATE statement that will only affect the rows I want.
Any advice on how to code this SQL?
UPDATE: I should have been more clear - I want to update the ID3 column to 9999 ONLY if the current value in ID3 is 1 ... AND there are no other rows that have the same ID1 and ID2 values and ID3 = 9999. ID1, ID2 and ID3 comprise a unique key and setting 9999 in ID3 can't duplicate a key value already in existence.

Answer 1

Maybe I'm reading that differently.
;WITH cte AS (
SELECT ID1, ID2
FROM tblConnector
GROUP BY ID1, ID2
HAVING COUNT(1) = 1
)
UPDATE tblConnector
SET ID3 = 9999
FROM tblConnector a
INNER JOIN
cte b ON a.ID1 = b.ID1
AND a.ID2 = b.ID2
sqlfiddle
EDIT (after comment)
Added your new sample records and WHERE ID3 = 1 to the UPDATE. In this case 1 is a placeholder for whatever single value you want to plugin.
;WITH cte AS (
SELECT ID1, ID2
FROM tblConnector
GROUP BY ID1, ID2
HAVING COUNT(1) = 1
)
UPDATE tblConnector
SET ID3 = 9999
FROM tblConnector a
INNER JOIN
cte b ON a.ID1 = b.ID1
AND a.ID2 = b.ID2
WHERE ID3 = 1
New fiddle

Answer 2

Following your exact wording, the following should work:
UPDATE tblConnector t1
SET ID3 = 9999
WHERE NOT EXISTS
(
SELECT 1
FROM tblConnector t2
WHERE t2.ID1 = t1.ID1
AND t2.ID2 = t1.ID2
AND t2.ID3 <> 9999
)

Answer 3

Without EXISTS, without CTE approach. I don't know if this will be most efficient.
update t1
set t1.id3 = 9999
--select *
from
tblConnector as t1
left join
(
select id1, id2, COUNT(id1) as Rowz
from tblConnector as t2
group by id1, id2
) as src
on t1.id1 = src.id1
where Rowz = 1