Question: Consider the SQL query select p a1 from p r1 r2
Consider the SQL query select p.a1 from p, r1, r2 where p.a1 = r1.a1 or p.a1 = r2.a1. Under what conditions does the preceding query select values of p.a1 that are either in r1 or in r2? Examine carefully the cases where one of r1 or r2 may be empty.
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