## Solving a relational query: part 3

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)

## Relational division again

Take the same relational database tables as before:

RESTAURANT(RT_ID, NAME, TYPE, LOCATION)
PROXIMITY(RT1_ID, RT2_ID, DISTANCE)
USER(U_ID, NAME, EMAIL)
REVIEW(U_ID, RT_ID, RT_DATE, RATING, COMMENT)

Find the names of the reviewers who reviewed all the restaurants in Earlsfield.

$\Pi _{NAME} ( USER \Join REVIEW ) \div \Pi _{RT\_ID} ( \sigma _{LOCATION = 'Earlsfield'} (RESTAURANT))$

But what about SQL? As I wrote of relational division when I started the blog:

one has to implement a “double not exists” query

In this case that means “find the name of any reviewer for whom there is no Earlsfield restaurant not reviewed by that reviewer”:

SELECT NAME
FROM USER
WHERE NOT EXISTS (
SELECT *
FROM RESTAURANT
WHERE LOCATION=’Earlsfield’ AND
NOT EXISTS (
SELECT *
FROM REVIEW
WHERE REVIEW.U_ID = USER.U_ID AND
REVIEW.RT_ID = RESTAURANT.RT_ID))

At least, I hope so! I am finding this all quite heavy going – and have ordered a book – SQL and Relational Theory: How to Write Accurate SQL Code – in the hope that it will help makes things clearer. Any other recommendations gratefully received.

Update: It does work, amazingly enough.