Relational division again

Relational divisionTake 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.

In relational algebra:

\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.

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