Test: Relational Model - Computer Science Engineering (CSE) MCQ

To understand the function of the given relational algebra expression, let's break it down:

σyear≥2009(book⋈borrow)

1. Natural Join (⋈): The expression book⋈borrow represents a natural join between the "book" and "borrow" relations. A natural join combines tuples from both relations based on common attributes.

2. Selection (σ): The selection operator σyear≥2009 filters the result of the natural join to include only those tuples where the "year" attribute is greater than or equal to 2009.

Given these points, the expression σyear≥2009(book⋈borrow) performs a natural join of the "book" and "borrow" relations first and then selects those tuples from the resulting set where the "year" attribute is greater than or equal to 2009.

The correct explanation is: Selects all the tuples from the natural join of book and borrow wherever the year is greater than or equal to 2009

We can specify a primary key in schema description by underlining the respective attribute with a bold line.

The “x” operator outputs all pairs of rows from the two input relations (regardless of whether or not they have the same values on common attributes). This operation is called as the Cartesian product operation and is similar to the Cartesian product of sets.

A foreign key is an attribute of a relation that is initially a primary key of another relation. A foreign key usage preserves referential integrity.

A superkey is an attribute or a set of multiple attributes that can uniquely identify a tuple. It is used to differentiate between tuples.

A tuple is a row is a relation. There can exist multiple foreign keys in the same relation because there can exist multiple attributes in the relation that are primary keys in two or more other relations.

An attribute is a column in a relation. A tuple is a row in a relation.

The ⋈ operator in relational algebra represents the natural join. It combines two relations based on attributes with the same name, producing pairs of rows where the values in those attributes match. The appropriate answer is:

Correct Answer: b)

Outputs pairs of rows from the two input relations that have the same value on all attributes that have the same name.

Explanation of Options:

• a) Describes the projection operator (π\piπ) that outputs specified attributes and removes duplicate tuples. This is incorrect for the natural join.
• b) Correctly describes the natural join (⋈\bowtie⋈), where rows are paired based on matching values in attributes with the same name.
• c) Describes the Cartesian product (×\times×), which pairs all rows from two relations regardless of attribute values. This is not the natural join.
• d) Describes the selection operator (σ\sigmaσ), which filters rows based on a condition. This is also incorrect for the natural join.

The state of the database system at any given point of time is called as an Instance of the database.

If a relation has a foreign key, then it is called a referencing relation of the foreign key dependency.