4.10.2.1 Relational databases Quiz: AQA Computer Science, Unit 10

The 20 questions

1. What is an attribute in a relational database table?

   • The name of the database that holds the relation together with its indexes, and which is stored separately from the relation on disk.
   • A rule that restricts which values may be entered into a table of records.
   • A named column in a table that holds one type of data for each row, such as Surname.
   • A single row that represents one record stored in the table of a database.

2. What is a primary key?

   • The first column of a table, which must always hold a text value of fixed length.
   • An attribute that links a table to another table in the same database system.
   • An attribute, or set of attributes, whose value uniquely identifies each row in a table.
   • A value that is allowed to repeat across rows so that searches can be made faster, because a repeated value is easier to index.

3. What is a composite primary key?

   • A primary key that combines the data from two different databases into one table.
   • A primary key that is made from a foreign key and an index in the same table.
   • A primary key that is generated automatically from the time each row was inserted into the table, using a clock value stored in the row.
   • A primary key made up of two or more attributes whose combined values are unique for each row.

4. What is a foreign key?

   • A primary key that is only allowed to hold values taken from a list of countries.
   • An attribute that is always numeric and never refers to any other table in the database.
   • An attribute in one table that refers to the primary key of another table.
   • An attribute that is stored in a separate file outside the database system entirely.

5. Which term describes a row in a relational table?

   • A domain, which is the set of values allowed in one column of the table.
   • A schema, which is the complete description of the structure of the database.
   • An attribute, which is a column that holds one data item for every record.
   • A tuple, which is one record in the relation.

6. What does the term relation refer to in a relational database?

   • A table, consisting of rows and columns that hold related data.
   • A query that combines several tables into one result set using a join condition.
   • A link between two separate database servers over a network connection in the cloud.
   • A rule that ensures that every foreign key value matches a primary key value somewhere.

7. In a Customer table with CustomerID as the primary key, which statement is true?

   • CustomerID may be left empty for some rows so that the key remains flexible.
   • CustomerID must be changed to a new value each time the customer places an order.
   • No two customers may have the same CustomerID value.
   • Two customers may share a CustomerID so long as they live at different addresses.

8. Which attribute would be the best primary key for a table of books?

   • Title, because book titles are always displayed in the library catalogue.
   • Price, because all books are priced differently from each other in the shop.
   • Author, because each author is listed only once in the library system.
   • ISBN, because each published book has a unique ISBN.

9. An Order table has CustomerID as a foreign key that refers to the Customer table. What does this ensure?

   • Customer rows are deleted automatically whenever their orders are removed.
   • Order rows are sorted by customer name whenever the table is displayed on screen.
   • Each order is linked to a customer who exists in the Customer table.
   • Every customer must place exactly one order, no more and no fewer than that.

10. A table of enrolments uses StudentID and CourseID together as its key. What type of key is this?

    • A surrogate key that is generated by the system for each new enrolment.
    • A composite primary key.
    • A candidate index that is never used to identify any row in the table.
    • A single foreign key that refers to a table of course descriptions.

11. In a table Enrolment (StudentID, CourseID, Grade), which attributes form the composite key?

    • StudentID and CourseID together.
    • Grade alone, because it changes for each student in each course.
    • CourseID alone, because each course has only one grade recorded for it.
    • StudentID alone, because every student identifier is always unique.

12. A table has no column whose values are unique for each row. What is the consequence?

    • The table must be split so that each row holds two primary keys at the same time.
    • The table has no primary key, so individual rows cannot be reliably identified.
    • The table can only be queried using a foreign key and not by any other attribute.
    • The database will automatically add a column of duplicate names to identify each row.

13. Which statement about foreign key values is correct?

    • A foreign key value must either match an existing primary key value in the referenced table or be empty, where empty values are allowed.
    • A foreign key value can be any text, because it is unrelated to the referenced table.
    • A foreign key is never allowed to be used in a join condition between two tables.
    • A foreign key must always be unique across the whole database, not only in its own table.

14. A Book table has ISBN as primary key, and a Loan table has ISBN as a foreign key. What does a loan with ISBN 978-1 imply?

    • The ISBN is a duplicate and must be removed from the Loan table before it is saved.
    • The book 978-1 has been deleted from the Book table, so the loan is invalid.
    • The Loan table is the primary key of the Book table, so the two tables are identical.
    • The loan refers to the book with ISBN 978-1, which must exist in the Book table.

15. A Teacher table has primary key TeacherID. A Class table also holds TeacherID. What is TeacherID in Class?

    • An attribute that is never allowed to hold any value at all in the table.
    • A composite primary key for the Class table that uses the teacher's name too.
    • A foreign key that refers to the Teacher table.
    • A duplicate of the Class table's name, stored to make searches faster.

16. Which attribute must be unique in a Student table with columns StudentID, Name and Form?

    • StudentID, because it is the primary key identifying each student.
    • Form, because each form group has a single list of students recorded in it.
    • Name, because each student's name is stored once only in the school system.
    • Name and Form together, because students of the same name are always in different forms.

17. Why does a relational database store data in separate linked tables rather than one large table?

    • So that every table can share the same primary key values without any restrictions.
    • To reduce repetition of data and to link related records through keys.
    • To store each attribute value in a separate file on the server, which is faster to search.
    • Because SQL cannot process any table that contains more than one column of data.

18. Evaluate: which statement best explains why a foreign key is useful?

    • It stores every value of the referenced table so that backups are not needed.
    • It makes every table in the database unique so that no duplicate rows ever exist.
    • It removes the need for a primary key in the related table, so storage is saved.
    • It keeps related records consistent, because each reference points to an existing record in another table.

19. A Phone table stores CustomerID and PhoneNumber. A customer may have several phone numbers. What is a suitable primary key for Phone?

    • PhoneNumber alone, which is never a foreign key that refers to another table.
    • A composite key of CustomerID and PhoneNumber, provided each number is unique for that customer.
    • A new column with no values, which is not used as a key in the table.
    • CustomerID alone, since each customer is assumed to have only one phone number.

20. A relation has attributes (Ticket, Seat, Event), and Ticket is unique for every row. Which statement is correct?

    • Ticket is a candidate key for the relation, so it could be used as the primary key.
    • No attribute can be a primary key, because the relation has three attributes only.
    • Event is the primary key, because it is the last attribute listed in the relation.
    • Seat must be the primary key, because it changes less often than the ticket does.

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