Question

Consider the following relational schema, representing a database of documents and a set of keywords:

Doc(did, docTitle)
Occurs(did, wid)
Word(wid, word)
Keywords(word)

For each of the queries below, show an equivalent relational algebra plan:

a.SELECT z.word, count(*)
FROM Doc x, Occurs y, Word z
WHERE x.did = y.did and y.wid = z.wid
GROUP BY z.wid, z.word

b.SELECT x.did, x.docTitle
FROM Doc x
WHERE NOT EXISTS (SELECT *
                  FROM Occurs o, Word w, Keyword k
                  WHERE x.did = o.did AND o.wid = w.wid AND w.word != k.word );

Answer 1

did docTitle DOC k keyword did-did AND wid-wid AND word!-word OCCUR word