_____clause is an additional filter that is applied to the result.
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.
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_____ command can be used to modify a column in a table
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.
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A weak entity has an ______ dependency on its owner entity, which can be used for both ON UPDATE and ON DELETE.
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.
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The ____ operator joins two or more conditions and displays rows only if that rows data satisfies all conditions specified.
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.
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If the closure of an attribute set is the entire relation then the attribute set is a
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.
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Whenever two independent one-to-many relationships are mixed in the same relation, a __________ arises.
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.
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Which of the following aggregate functions does not ignore nulls in its results?
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.
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The part of a database management system which ensures that the data remains in a consistent state is
Answer: C
Correct answer: transaction manager
The transaction manager ensures that every transaction takes the database from one consistent state to another, committing all of its changes or none of them. The buffer and file managers move pages, and the integrity manager checks constraints on individual operations.
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______defines the structure of a relation which consists of a fixed set of attribute-domain pairs.
Answer: C
Correct answer: Schema
A schema is the description of a relation's structure: its attributes and the domain of each. An instance is the set of tuples present at a given moment, which changes while the schema stays fixed.
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Which normal form is considered adequate for normal relational database design?
Answer: D
Correct answer: 3NF
Third normal form removes both partial and transitive dependencies and can always be reached without losing any functional dependency, which is why it is the normal form ordinarily aimed at. BCNF is stronger but may not be dependency preserving, and 4NF and 5NF address problems that arise less often.
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