How does that contradict my statement? The BookAuthors table seems to contain one entry for each pair (book, author), so (in a contrived example) it could look like the following:
book author
--------------------
1 Jones
1 Miller
2 Jones
2 Smith
3 Smith
4 Jones
The statement will return every book id for the given query of author IN ('Jones', 'Smith'), because, as you say, IN is more or less equivalent to a series of OR statements (I'm not sure about differences regarding NULL values).
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