Database Management System Questions and Answers

Answer: A

Correct answer: 4,0

The difference \( A - B \) keeps A's columns, so the degree is still 4. It returns the tuples of A that are not in B, and every row of A is said to appear in B, so no rows remain.

Answer: D

Correct answer: Domain

The domain of an attribute is the set of values it may take - integers in a range, a list of permitted codes, dates. Cardinality is the number of tuples and degree the number of attributes.

Answer: C

Correct answer: Advanced filter

Access offers Filter by Selection and Filter by Form as its two filtering commands. Its third facility is Advanced Filter/Sort, which opens a query-like grid and both filters and sorts - not a plain advanced filter alongside the other two.

Answer: D

Correct answer: Having

HAVING filters the groups produced by GROUP BY, after aggregation. WHERE filters rows before they are grouped, which is why a condition on an aggregate such as \( \text{COUNT}(*)>5 \) must go in HAVING and not in WHERE.

Answer: A

Correct answer: alter

ALTER TABLE changes the structure - adding, dropping or modifying a column. UPDATE changes the data inside existing columns, which is a different operation.

Answer: C

Correct answer: Existence

A weak entity cannot exist without its owner: a dependant without an employee, an order line without an order. That existence dependency is why deleting the owner must delete the weak entity too, which is what the ON DELETE action expresses.

Answer: B

Correct answer: AND

AND requires every condition to be satisfied, so a row appears only if all of them hold. OR would admit a row satisfying any one of them.

Answer: A

Correct answer: superkey

If the closure of an attribute set determines every attribute of the relation, that set identifies each tuple uniquely, which is the definition of a superkey. It is a candidate key only if no proper subset of it also determines the whole relation.

Answer: B

Correct answer: Multi-valued dependency

If one key has two independent one-to-many relationships in the same table, every combination of the two sets of values must be stored, and the two sets are unrelated to each other. That is a multi-valued dependency, and removing it is what fourth normal form is for.

Answer: B

Correct answer: COUNT (*)

Every aggregate ignores nulls, because a null is not a value to be counted or summed. COUNT(*) is the exception: it counts rows rather than values, so a row consisting entirely of nulls is still counted.