Lecture 3: Relational Model

Question 1

Explain what a relational Model is.

Answer 1

• in a relational model all data is structured within relations (tables).
• this is the dominant database solution

Question 2

So what is a relation?

Answer 2

table with columns and rows

Question 3

What is an attribute?

Answer 3

a column of a relation

Question 4

What is a tuple?

Answer 4

a row in a relation.

Question 5

what is a domain?

Answer 5

set of allowable values for one or more attributes.

Question 6

Explain the term degree (of a relation).

Answer 6

number of attributes it contains

Question 7

Describe Cardinality (of a relation)

Answer 7

number of tuples it contains.

Question 8

In a relational model we have constraints that impose restrictions on values in a column or a combination of columns within a table.

What is a constraint?

Answer 8

a column or a group of columns must be unique across all rows in the table.

Question 9

Explain what a: Superkey, Candidate Key and Primary Key is.

Answer 9

Superkey: attribute or set of attributes that uniquely identify a typle in a relation.

Candidate Key: a superkey such that none if its subset is a superkey for that relation. e.g.

• its unique
• itrs irreducible.

Primary Key: the candidate key that is used to uniquely identify tupes in a relation

• thus, there may be candidate keys, but only one is chosed as PK

Question 10

Explain what a Foreign Key is.

Answer 10

• an attribute or a set of attributes within one relation (home relation), that matches the candidate key in another relation.
• The foreigner key targets the candidate key in the home relation.

Question 11

Explain the use of Null

Answer 11

• represents a value that is unknown or the absence of a value
• null can be used when we dont know the value for an attribute to prevent inserting a false value.
• stock = 0, stock = null. is not the same

Question 12

Name and provide a description of the two Integrity constraints.

Answer 12

Entity integrity: no attribute of a primary key can be null

Referential integrity: if a foreign key exists, either the value of the foreign key matches the value of the candidate key in the home relation or the value of the foreign key is null.

Question 13

Give the example of how data can be modeled from different perspectives.

Answer 13

Relational Perspective (How the Users view the data):

• Table, Row, Column

Conceptual Perspective:

• Relation, Tuple, Attribute

Storage Perspective (Describes the details of how the data is stored in the computer):

• File, Record, Field

Question 14

Explain the constraints:

• Entity Integrity
• Referential integrity
• Domain Constraints
• Primary Key Constraints

Answer 14

Referential Integrity: if a foreign key exist, either the value of the foreign key matches the value of the candidate key in the home relation or the value of the foreign key is NULL

Entity integrity: no attribute of a primary key can be NULL

Domain Constraints: the value of each attribute in each tuple must be from the domain assigned to this attribute

Primary Key Constraints: two distinct tuples in any state of the relation cannot have identical values for all attributes in the key