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12.4 RELATIONAL DATABASE

A relational database is a type of database that stores and
provides access to data points that are related to one another.
Relational databases are based on the relational model, an
intuitive, straightforward way of representing data in tables. In
a relational database, each row in the table is a record with a
unique ID called the key. The columns of the table hold attributes
of the data and each record usually has a value for each attribute,
making it easy to establish the relationships among data points.
Fig. 12.10: Tuples and Attributes
Often, data in a relational database is organized into tables.
Basic Terminologies related to a Relational Database

1. Entity: An entity is something that exists and about which we can store some information. It is
an object which can be distinctly identified. For example, student entity, employee entity, item
entity, etc. Entity becomes the name of the table.
2. Attribute: In a relational table, an attribute is a set of values of a particular type. The term attribute
is also used to represent a column. A table consists of several records (row); each record can be
broken into several smaller entities known as fields or attributes or columns. A set of attributes
defines the characteristics or properties of an entity. In the given table, Student relation (Fig.
12.11) consists of four fields or attributes—Roll number, Name, Address and Gender.
3. Tuple: Each row in a table is known as tuple. It is also called a row/record. A single entry in a
table is called a record or row. A record in a table represents a set of related data. For example,
the Student table given below has 10 records.

4. Cardinality of Relation: It is the number of records or tuples in the relation. Thus, the
cardinality of Student relation is 10.

5. Degree of Relation: Number of columns or attributes is known as degree of a relation. Thus,
the degree of Student relation is 4.
6. Domain of Relation: It defines the kind of data represented by the attribute. It is the set of all
possible values that an attribute may contain. For example, in the given table Student, domain
for the field Gender is two since it can have either ‘M’ or ‘F’ as the possible and available values
that it may contain.
7. Body of the Relation: It consists of an unordered set of 0 or more tuples.

Relational Database and SQL

Fig. 12.11: Relational Model (Student Relation)
12.9