Let's say I have the following statement and the inner join results in 3 rows where a.Id = b.Id, but each of the 3 rows have different b.Value's. Since only one row from tableA is being updated, which of the 3 values is used in the update?
UPDATE a
SET a.Value = b.Value
FROM tableA AS a
INNER JOIN tableB as b
ON a.Id = b.Id
I don't think there are rules for this case and you cannot depend on a particular outcome.
If you're after a specific row, say the latest one, you can use apply
, like:
UPDATE a
SET a.Value = b.Value
FROM tableA AS a
CROSS APPLY
(
select top 1 *
from tableB as b
where b.id = a.id
order by
DateColumn desc
) as b