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I agree that this is often the use case but depending on how the query (as well as how sparse the nulls are, the total data size and partitioning scheme I guess) it's also possible to achieve this through a join-table without introducing NULL values.

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Unless you outer join, in which case NULL values pop up out of thin air...

Good point. I did try to say that whether that's a viable technique would depend on your query (but omitted some verb that should have made it better resemble a sentence in English :)).

But yes, even if no nulls are present in any table, outer join could (or maybe even should) introduce them.

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