Log analytics or warehouse tables often have no simple useful key for this sort of comparison.
Also in a more general case you might be comparing tables that may contain the same data but have been constructed from different sources. Or perhaps a distributed dataset became disconnected and may have seen updates in both partitions, and you have brought them together to compare to try decide which to keep or if it is worth trying to merge. In those and other circumstances there may be a key but if it is a surrogate key it will be meaningless for comparing data from two sets of updates, so you would have to disregard it and compare on the other data (which might not include useful candidate keys).
Also in a more general case you might be comparing tables that may contain the same data but have been constructed from different sources. Or perhaps a distributed dataset became disconnected and may have seen updates in both partitions, and you have brought them together to compare to try decide which to keep or if it is worth trying to merge. In those and other circumstances there may be a key but if it is a surrogate key it will be meaningless for comparing data from two sets of updates, so you would have to disregard it and compare on the other data (which might not include useful candidate keys).