A DBMS that is based on relational model is called as RDBMS. Relation model is most
successful mode of all three models. Designed by E.F. Codd, relational model is based
on the theory of sets and relations of mathematics.
Relational model represents data in the form a table. A table is a two dimensional
array containing rows and columns. Each row contains data related to an entity such
as a student. Each column contains the data related to a single attribute of the entity
such as student name.
One of the reasons behind the success of relational model is its simplicity. It is easy to
understand the data and easy to manipulate.
Another important advantage with relational model, compared with remaining two
models is, it doesn’t bind data with relationship between data item. Instead it allows
you to have dynamic relationship between entities using the values of the columns.
Almost all Database systems that are sold in the market, now- a-days, have either
complete or partial implementation of relational model.
Tuple / Row : A single row in the table is called as tuple. Each row represents the data of a
single entity.
Attribute / Column : A column stores an attribute of the entity. For example, if details of students are
stored then student name is an attribute; course is another attribute and so on.
Column Name :Each column in the table is given a name. This name is used to refer to value in the
column.
Table Name : Each table is given a name. This is used to refer to the table. The name depicts the
content of the table.
The following are two other terms, primary key and foreign key, that are very
important in relational model.
Primary Key: A table contains the data related entities. If you take STUDETNS table, it contains data
related to students. For each student there will be one row in the table. Each student’s
data in the table must be uniquely identified. In order to identify each entity uniquely
in the table, we use a column in the table. That column, which is used to uniquely
identify entities (students) in the table is called as primary key.
In case of STUDENTS table (see figure 1) we can use ROLLNO as the primary key as it
in not duplicated.
So a primary key can be defined as a set of columns used to uniquely
identify rows of a table.
Composite Primary Key : In some tables a single column cannot be used to uniquely identify entities (rows). In
that case we have to use two or more columns to uniquely identify rows of the table.
When a primary key contains two or more columns it is called as composite primary
key.
we have PAYMENTS table, which contains the details of payments made by
the students. Each row in the table contains roll number of the student, payment date
and amount paid. Neither of the columns can uniquely identify rows. So we have to
combine ROLLNO and DP to uniquely identify rows in the table. As primary key is
consisting of two columns it is called as composite primary key.
Foreign Key: In relational model, we often store data in different tables and put them together to
get complete information. For example, in PAYMENTS table we have only ROLLNO of
the student. To get remaining information about the student we have to use
STUDETNS table. Roll number in PAYMENTS table can be used to obtain remaining
information about the student.
The relationship between entities student and payment is one-to-many. One student
may make payment for many times. As we already have ROLLNO column in PAYMENTS
table, it is possible to join with STUDENTS table and get information about parent
entity (student).
Roll number column of PAYMENTS table is called as foreign key as it is used to join
PAYMENTS table with STUDENTS table. So foreign key is the key on the many side of
the relationship.
ROLLNO column of PAYMENTS table must derive its values from ROLLNO column of
STUDENTS table.
When a child table contains a row that doesn’t refer to a corresponding parent key, it
is called as orphan record. We must not have orphan records, as they are result of lack
of data integrity.
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