>Regarding the query below, the value "A&P" is not in the acctnrlu table (in the luacctnm column). A&P is in curdata. So the query should return rows. However, it wasn't returning anything. There was one row with a null in acctnrlu.luacctnm. I didn't think this should matter. Yet when I removed the row with the null, the query began working. Is this the expected behavior?
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>SELECT accountnam FROM curdata WHERE accountnam NOT in (SELECT luacctnm FROM acctnrlu)

Yes, this is the expected behavior. See this strong blog post
Why you should never use IN/NOT IN in SQL