A library relational database system uses the following schema USERS (User#,…
1996
A library relational database system uses the following schema
USERS (User#, UserName, HomeTown)
BOOKS (Book#, BookTitle, AuthorName)
ISSUED (Book#, User#, Date)Explain in one English sentence, what each of the following relational algebra queries is designed to determine
Answer: A. (a) Book titles of books issued by User# 6; (b) author names of books issued by users whose HomeTown is Delhi. — For query (a), the expression joins USERS, ISSUED, and BOOKS, keeps the attributes User# and BookTitle, and then selects User# = 6. Therefore, it determines…
- A.
(a) Book titles of books issued by User# 6; (b) author names of books issued by users whose HomeTown is Delhi.
- B.
(a) Author names of books issued by users from Delhi; (b) book titles issued by User# 6.
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Correct answer: A
For query (a), the expression joins USERS, ISSUED, and BOOKS, keeps the attributes User# and BookTitle, and then selects User# = 6. Therefore, it determines the titles of books issued by User# 6.
For query (b), the expression first joins USERS with ISSUED and selects users whose HomeTown is Delhi. It then joins the result with BOOKS and projects AuthorName. Therefore, it determines the names of authors of books issued by users from Delhi.
Hence the correct explanation is: (a) book titles issued by User# 6; (b) author names of books issued by users whose HomeTown is Delhi.