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SELECT Istream(AVG(Q.final)) 

FROM Category C, (

 SELECT Rstream(MAX(B.price) AS final, A.category)

 FROM Auction A [ROWS UNBOUNDED], Bid B [ROWS UNBOUNDED]

 WHERE A.id=B.auction AND B.datetime < A.expires AND A.expires < CURRENT_TIME GROUP BY A.id, A.category) Q 

WHERE Q.category = C.id GROUP BY C.id;