thanks alot guys i really learned quite a bit and appreciate the help. drew.allen i was headed in that direction via chris' info

okay that went over my head:
Use ROW_NUMBER() to number the rows in table b in a CTE, then join to table a - either on table a id, or...

I'm trying to sequentially get the row of id's from TableB.id into TableA.tableid from row 1 to row 4512

TableB
 id
  123
  456

end results to...

i tried this:

UPDATE TableA
SET tablebid = t1.id
FROM TableB t1
where t1.id IN (
SELECT NULL AS id FROM TableA WHERE tableid IS NOT null
UNION

i don't have a way to join the two tables as TableA.tablebid is null.
select tablebid,null as id from TableA
union
select null,id from TableB

results were like: