Relational Database क्या है? RDBMS और SQL पूरी जानकारी हिंदी में

A Relational Data Model stores data in the form of tables (relations). Each table consists of rows (tuples) and columns (attributes).

Definition

Relational Data Model एक ऐसा Database Model है जिसमें Data को Table (Relation) के रूप में Store किया जाता है।

हर Table में:

• Rows (Records/Tuples) होते हैं।
• Columns (Attributes) होते हैं।

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Real-Life Example

Imagine a School Database.

Instead of storing all information in one large file, we organize it into tables.

Student Table

StudentID	Name	Age	Department
101	Rahul	20	CSE
102	Aman	21	IT
103	Priya	20	ECE

Course Table

CourseID	CourseName
C101	DBMS
C102	Operating System
C103	Computer Networks

Enrollment Table

StudentID	CourseID
101	C101
101	C102
102	C101

Notice that we don't repeat the student's full information in every row. Instead, we use StudentID and CourseID to create relationships.

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Why Use the Relational Model?

Without a relational model:

❌ Duplicate data

❌ Difficult searching

❌ Hard maintenance

❌ Data inconsistency

With a relational model:

✔ Organized data

✔ Easy SQL queries

✔ Less redundancy

✔ Better security

✔ Data integrity

✔ Easy relationships

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Main Components of the Relational Model

           Relational Model

                │
    ┌───────────┼────────────┐
    │           │            │
 Relation    Attribute     Tuple
    │           │            │
 Domain      Keys      Constraints

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Relation (Table)

A Relation is a table in a relational database.

Each relation has:

• Table Name
• Columns
• Rows

Example

Student

StudentID	Name	Age
101	Rahul	20
102	Aman	21
103	Priya	19

Here:

• Relation = Student Table

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Properties of a Relation

A relation follows these rules:

1. Unique Table Name

Every table must have a unique name.

✔ Student

✔ Employee

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2. Unique Column Names

No duplicate attribute names.

✔ Name

✔ Age

❌ Age, Age

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3. No Duplicate Rows

Every record must be unique.

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4. Order Doesn't Matter

Changing row order doesn't change the table.

Original:

StudentID	Name
101	Rahul
102	Aman

Reordered:

StudentID	Name
102	Aman
101	Rahul

Both represent the same relation.

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5. Atomic Values

Each cell contains only one value.

✔ Phone = 9876543210

❌ Phone = 9876543210, 9988776655

Multiple values should be stored in a separate related table.

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Tuple (Row / Record)

A Tuple is a single row in a relation.

Example

StudentID	Name	Age
101	Rahul	20

This entire row is one Tuple.

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 Attribute (Column)

An Attribute is a column in a table.

Example

|StudentID|Name|Age|

Attributes are:

• StudentID
• Name
• Age

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Domain

A Domain is the set of valid values that an attribute can contain.

Example

Attribute: Age

Valid Domain: 

18–60

If someone enters:

Age = 250

❌ Invalid (outside the defined domain)

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Another example:

Gender

Allowed values: 

Male

Female

Other

The domain restricts invalid data.

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 Degree of a Relation

The Degree is the number of attributes (columns) in a table.

Example

|StudentID|Name|Age|Department|

Number of columns = 4

Degree = 4

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 Cardinality of a Relation

The Cardinality is the number of tuples (rows) in a table.

Example

StudentID	Name
101	Rahul
102	Aman
103	Priya
104	Neha

Rows = 4

Cardinality = 4

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Degree vs Cardinality

Degree	Cardinality
Number of Columns	Number of Rows
Usually fixed	Changes as records are inserted or deleted

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 Schema

A Schema is the logical structure or design of a database.

It describes:

• Table names
• Columns
• Data types
• Relationships
• Constraints

Example 

Student(
    StudentID INT,
    Name VARCHAR(50),
    Age INT,
    Department VARCHAR(20)
)

This is the Schema.

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Database Schema 

College Database

│

├── Student

├── Teacher

├── Course

└── Enrollment

This overall design is the database schema.

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Instance

An Instance is the actual data stored in the database at a specific time.

Schema vs Instance

Schema:

Student(
StudentID,
Name,
Age
)

Instance:

StudentID	Name	Age
101	Rahul	20
102	Aman	21

The schema remains mostly unchanged, while the instance changes as data is added, updated, or deleted.

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Schema vs Instance

Schema	Instance
Structure	Actual Data
Changes rarely	Changes frequently
Blueprint	Current content

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Null Value

A NULL means that the value is unknown, unavailable, or not applicable.

Example

StudentID	Name	Phone
101	Rahul	9876543210
102	Aman	NULL

NULL does not mean:

• Zero (0)
• Empty string ("")
• False

It simply means the value is missing or unknown.

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 Relational Constraints

Constraints ensure that data remains accurate and consistent.

There are four main constraints.

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 1. Domain Constraint

Ensures values belong to the defined domain.

Example:

Age must be between 18 and 60.

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2. Key Constraint

Primary Key values must be unique.

Example:

StudentID	Name
101	Rahul
101	Aman

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3. Entity Integrity Constraint

A Primary Key:

• Cannot be NULL
• Must be unique

Example:

StudentID	Name
NULL	Rahul

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4. Referential Integrity Constraint

A Foreign Key value must exist in the referenced table.

Student

StudentID	Name
101	Rahul

Enrollment

StudentID	CourseID
101	C101
999	C101

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Advantages of the Relational Model

• Easy to understand.

• Table-based structure.
• Supports SQL.
• Reduces redundancy.
• Strong data integrity.
• Better security.
• Easy maintenance.
• Flexible design.

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Disadvantages

• Large joins can affect performance.
• Not ideal for some highly complex or graph-like relationships.
• Proper normalization is required for good design.

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Example

Employee Table

EmpID	Name	Department	Salary
E101	Rahul	IT	50000
E102	Aman	HR	45000
E103	Neha	Finance	60000

Analysis:

• Relation → Employee
• Attributes → EmpID, Name, Department, Salary
• Tuple → Any single row
• Degree → 4
• Cardinality → 3
• Domain → Salary must be a valid number
• Primary Key → EmpID