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The table Arc(x,y) currently has the following tuples (note there are duplicates):
(1,2), (1,2), (2,3), (3,4), (3,4), (4,1), (4,1), (4,1), (4,2) Compute the result of the query:
SELECT a1.x, a2.y, COUNT(*) FROM Arc a1, Arc a2 WHERE a1.y = a2.x GROUP BY a1.x, a2.y; What are a1 and a2 referring to?
a1 and a2 are just aliases for the Arc table which is being joined to itself. You could also say,
Is that what you’re asking?