Thanks for all your help so far.

This is one way I've tried and it works I think but I'm sure you clever folks can come up with a better way :

declare @pairs table (dropped_id int, retained_id int)

insert into @pairs

SELECT 34336, 14553 UNION ALL

SELECT 39352, 14553 UNION ALL

SELECT 39352, 34336 UNION ALL

SELECT 39776, 34123 UNION ALL

SELECT 47833, 14553 UNION ALL

SELECT 47833, 34336 UNION ALL

SELECT 47833, 39352 UNION ALL

SELECT 47969, 14553 UNION ALL

SELECT 47969, 34336 UNION ALL

SELECT 47969, 39352 UNION ALL

SELECT 47969, 47833 UNION ALL

SELECT 48460, 14553 UNION ALL

SELECT 48460, 34336 UNION ALL

SELECT 48460, 39352 UNION ALL

SELECT 48460, 47833 UNION ALL

SELECT 48460, 47969 UNION ALL

SELECT 48463, 14553 UNION ALL

SELECT 48463, 34336 UNION ALL

SELECT 48463, 39352 UNION ALL

SELECT 48463, 47833 UNION ALL

SELECT 48463, 47969 UNION ALL

SELECT 48463, 48460 UNION ALL

SELECT 52149, 14553 UNION ALL

SELECT 52149, 34336 UNION ALL

SELECT 52149, 39352 UNION ALL

SELECT 52149, 39776 UNION ALL

SELECT 52149, 47833 UNION ALL

SELECT 52149, 47969 UNION ALL

SELECT 52149, 48460 UNION ALL

SELECT 52149, 48463 UNION ALL

SELECT 54497, 14553 UNION ALL

SELECT 54497, 34336 UNION ALL

SELECT 54497, 39352 UNION ALL

SELECT 54497, 39776 UNION ALL

SELECT 54497, 47833 UNION ALL

SELECT 54497, 47969 UNION ALL

SELECT 54497, 48460 UNION ALL

SELECT 54497, 48463 UNION ALL

SELECT 54497, 52149 UNION ALL

SELECT 66316, 14553 UNION ALL

SELECT 66316, 34336 UNION ALL

SELECT 66316, 39352 UNION ALL

SELECT 66316, 47833 UNION ALL

SELECT 66316, 47969 UNION ALL

SELECT 66316, 48460 UNION ALL

SELECT 66316, 48463 UNION ALL

SELECT 66316, 48819 UNION ALL

SELECT 66316, 52149 UNION ALL

SELECT 66316, 54497 UNION ALL

SELECT 74054, 66316 UNION ALL

SELECT 77722, 53333

declare @sortedpairs table (retained_id int, dropped_id int)

insert into @sortedpairs (retained_id,dropped_id )

select retained_id ,dropped_id from @pairs where retained_id <dropped_id

insert into @sortedpairs (retained_id,dropped_id )

select dropped_id ,retained_id from @pairs where retained_id >dropped_id

insert into @sortedpairs (retained_ID,dropped_ID)

select sp1.retained_ID,sp2.retained_ID from @sortedpairs sp1

inner join @sortedpairs sp2

on sp1.dropped_ID=sp2.dropped_ID

and sp1.retained_ID<>sp2.retained_ID

and sp1.retained_ID<sp2.retained_ID

insert into @sortedpairs (dropped_ID,retained_ID)

select sp1.retained_ID,sp2.retained_ID from @sortedpairs sp1

inner join @sortedpairs sp2

on sp1.dropped_ID=sp2.dropped_ID

and sp1.retained_ID<>sp2.retained_ID

and sp1.retained_ID>sp2.retained_ID

while (@@rowcount<>0)

begin

update tab1 set tab1.retained_ID=tab2.retained_ID

from @sortedpairs tab1

inner join @sortedpairs tab2

on tab2.dropped_ID=tab1.retained_ID

end

select * from @sortedpairs

group by retained_ID,dropped_ID

order by 1