Introduction To Database Questions and Answers

Answer: A

Correct answer: mn

A join is a selection applied to the Cartesian product, so it can never return more tuples than the product itself, which has \(m \times n\). That maximum is reached only when the join condition matches every pair - in effect a cross join. A typical equijoin on a foreign key returns far fewer.

Answer: C

Correct answer: Combination of selection and Cartesian product

A natural join is built from the basic operators: form the Cartesian product of the two relations, then select the tuples whose common attributes are equal - and finally project away the duplicated columns. That is why the join is a derived operation rather than a primitive of relational algebra.

Answer: C

Correct answer: View never contains derived columns.

That statement is false: a view may perfectly well define computed columns, such as price * quantity AS total, and doing so is one of its common uses. The other three are true - a view is derived from other tables, it is virtual in the sense that it stores no rows, and its definition is stored permanently in the system catalogue.

Answer: A

Correct answer: Parent-Child relationship between the tables that connect them.

A foreign key must match a primary key in another table, which makes that table the parent and this one the child. The result is a one-to-many relationship, and it lets the DBMS enforce referential integrity: no child row may refer to a parent that does not exist.

Answer: D

Correct answer: Diamond

In the standard E-R notation, a diamond is a relationship type, a rectangle an entity set, and an ellipse an attribute. A double diamond marks the identifying relationship of a weak entity.

Answer: A

Correct answer: Tree structure

The hierarchical model organises records into a tree: every record type except the root has exactly one parent, and access is by navigating down from the root. That single-parent rule is its limitation - a many-to-many relationship cannot be represented without duplicating data - and it is what the network model, a “plex” or graph structure, was devised to relax.

Answer: D

Correct answer: Triangle

Generalisation and its inverse, specialisation, are drawn as a triangle labelled ISA, connecting the general entity set above to the specialised ones below. An ellipse is an attribute, a dashed ellipse a derived one, and a rectangle an entity set.

Answer: B

Correct answer: Hash

Hashing applies a transformation to the key value to compute the address of the record directly, so a lookup takes one access with no index to search. The price is that a hash file supports no range queries and needs a strategy for collisions.

Answer: B

Correct answer: Self Join

A self join joins a table to itself, using two different aliases so that the two copies can be distinguished. It is how a table that refers to its own rows is unravelled - listing each employee beside their manager, where both are rows of the same employee table.

Answer: A

Correct answer: Tree

The hierarchical model is based on the tree: one root, and every other record type with exactly one parent. Its successor, the network model, allows a record to have several parents and so is based on a graph.