The Relational Model: Relation, Tuple, Attribute, Domain, Degree and Cardinality

Hello students, welcome to Kwickprep. Your class register is a simple table. But in database language, the table, its rows and its columns all have special names. Today we will learn those names, count degree and cardinality, and compare three ways of organising a database.

First, a new term. A data model is a plan that decides how data is organised and linked inside a database. The relational model keeps all data in tables made of rows and columns. Two tables are linked when they share a common column, like a roll number. This model was proposed by Edgar Codd in nineteen seventy, and My S Q L, Oracle and most databases today follow it.

Here is a small table called Student. Riya has roll number one and studies in ten A. Aman has roll number two and studies in ten B. Neha has roll number three and studies in ten A. Kabir has roll number four and studies in ten C. The top line, with Roll No, Name and Class, is the heading.

Now let us label the parts of the table. The whole table is called a relation, so Student is a relation. One row is called a tuple, so the row two, Aman, ten B is one tuple. One column is called an attribute, so Name is an attribute. Remember, a tuple is one complete record, and an attribute is one property of that record.

Next comes domain. The domain of an attribute is the set of all values that the attribute is allowed to hold. Every value in a column must come from its domain. If a value falls outside the domain, it is invalid data, and a good database refuses it.

Let us write the domain of each attribute in Student. Roll No takes whole numbers from one to sixty, so minus five or seven point five is invalid. Name takes text made of letters, so the number one two three four five makes no sense. Class takes only the sections that exist, ten A, ten B and ten C, so ten Z is invalid. Notice that domain is about values, not about how many rows there are.

Two counting words come up in almost every exam. Degree is the number of attributes, which means the number of columns. Cardinality is the number of tuples, which means the number of data rows. Be careful, the heading row is not a tuple, so never count it.

Let us count together, one by one. Count the columns: Roll No is one, Name is two, Class is three. So the degree of Student is three. Now count the rows: Riya is one, Aman is two, Neha is three, Kabir is four. So the cardinality of Student is four.

Pause and predict before the answer appears. What changes if a new student joins, and what changes if we add a City column? When Meera joins, one tuple is added, so cardinality goes from four to five. When we add a City column, one attribute is added, so degree goes from three to four. Adding rows changes only cardinality, and adding columns changes only degree.

A relation follows a few simple rules. No two tuples can be exactly the same. The order of the rows does not matter, so sorting by name gives the same relation. Every attribute in a table has a different name. And each cell holds just one value, not a list of values.

Some courses also compare three data models. The hierarchical model arranges data as a tree, like a school with classes, and students inside each class. The network model arranges data as a web of links, like trains and stations, where one station serves many trains. The relational model, which we studied today, keeps data in tables.

Here is the key difference between them. In the hierarchical model, each child record has only one parent, so one student sits under one class. In the network model, a record can have many parents, so one station can link to many trains. In the relational model, there are no fixed links, and tables connect through a common column. That flexibility makes the relational model the easiest and the most widely used today.

Let us revise what we learned today. A relation is a table, a tuple is a row, and an attribute is a column. The domain of an attribute is its set of allowed values. Degree counts the columns, and cardinality counts the data rows. Hierarchical models are trees, network models are webs, and relational models are tables. Draw any table from your notebook and find its degree and cardinality.