Solving a relational query: part 3


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This is starting to depress me, now, because I thought I was full on top of all this material until I tried this question.

So, going back to the database specified in part 1 – I now need to find the relational algebra (not the SQL, I have done that) for: “find the name and email of any user who has made multiple bids for a lot including a bid exceeding £100”.

Various ideas have crossed my mind and some bits are easy eg USER \Join \sigma _{AMOUNT > 100} \rho (BIDDER\_ID -> USER\_ID,\ BID) gets a relation from which we coul;d extract users who bid more than £100 for anything, but how can I get it all to string together.

I think the SQL that works is:

SELECT DISTINCT U.NAME, U.EMAIL
FROM USER U, BID B, LOT L
WHERE U.USER_ID = B.BIDDER_ID AND
B.AMOUNT > 100 AND B.LOT_ID = L.LOT_ID
AND L.LOT_ID IN (
SELECT COUNT(*) > 1 FROM
LOT, BID WHERE
LOT.LOT_ID = L.LOT_ID AND BID.LOT_ID = LOT.LOT_ID
AND BID.BIDDER_ID = U.USER_ID)