The HotelReviews.com database consists of three relations: HOTELS(HID,Category,Price), REVIEWERS(RID,AGE), REVIEWS(RID, HID,SCORE). Underlined attributes represent keys for...

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The "HotelReviews.com" database consists of three relations: HOTELS(HID,Category,Price), REVIEWERS(RID,AGE), REVIEWS(RID, HID,SCORE). Underlined attributes represent keys for the respective relations. Relation REVIEWS has a key that consists of two attributes, RID and HID. Attributes RID and HID in REVIEWS are also foreign keys, w.r.t. attribute RID in relation REVIEWERS and attribute HID in relation HOTELS. Attribute "score" in relation REVIEWS is always a strictly positive integer. The same apply to attribute "age" in relation REVIEWERS and attribute "price" in relation HOTELS. The tables below show a plausible instance of the database: | HOTELS | REVIEWERS | I ridage | |- hid | category | price 320 | Ada 189 | Bob 179 Tom | 99 | Zoe | Astoria 5-stars 4-stars | Belvedere Vertigo | 4-stars. Laguna | 3-stars | 33 | 21 | 25 21 REVIEWS rid I | Ada | | Ada Bob | | Bob | | Zoe | hid score Belvedere i Vertigo | Laguna | Vertigo I Belvedere | Exercise 3) [12 points] With respect to the database instance presented above, a) Draw the relation returned by the following relation algebra expression: R3:= THID (HOTELS) - THID (REVIEWS) b) Explain in your own words what is returned by the expression above c) Translate the definition of R3 into a (tuple-) relational calculus expression. 4 41 51 The "HotelReviews.com" database consists of three relations: HOTELS(HID,Category,Price), REVIEWERS(RID,AGE), REVIEWS(RID, HID,SCORE). Underlined attributes represent keys for the respective relations. Relation REVIEWS has a key that consists of two attributes, RID and HID. Attributes RID and HID in REVIEWS are also foreign keys, w.r.t. attribute RID in relation REVIEWERS and attribute HID in relation HOTELS. Attribute "score" in relation REVIEWS is always a strictly positive integer. The same apply to attribute "age" in relation REVIEWERS and attribute "price" in relation HOTELS. The tables below show a plausible instance of the database: | HOTELS | REVIEWERS | I ridage | |- hid | category | price 320 | Ada 189 | Bob 179 Tom | 99 | Zoe | Astoria 5-stars 4-stars | Belvedere Vertigo | 4-stars. Laguna | 3-stars | 33 | 21 | 25 21 REVIEWS rid I | Ada | | Ada Bob | | Bob | | Zoe | hid score Belvedere i Vertigo | Laguna | Vertigo I Belvedere | Exercise 3) [12 points] With respect to the database instance presented above, a) Draw the relation returned by the following relation algebra expression: R3:= THID (HOTELS) - THID (REVIEWS) b) Explain in your own words what is returned by the expression above c) Translate the definition of R3 into a (tuple-) relational calculus expression. 4 41 51

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