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 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)
Update: See part 4 (next post)
- Solving a relational query: part 1 (cartesianproduct.wordpress.com)
- Solving a relational query: part 2 (cartesianproduct.wordpress.com)
- Relational algebra problem (cartesianproduct.wordpress.com)
- A co-Relational Model of Data for Large Shared Data Banks – ACM Queue (queue.acm.org)