Keys in DBMS

 Keys ka use kyun hota hai?

DBMS (Database Management System) mein keys ka use data ko uniquely identify karne aur tables ke beech relation banane ke liye hota hai. Keys se:

• Hum records ko unique tarike se pehchaan sakte hain

• Tables ke beech relationship banate hain

• Duplicate data se bachav hota hai

• Data consistency maintain hoti hai

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 Types of Keys in DBMS (Simple Examples ke saath)

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1. Primary Key

• Ye table ka main key hoti hai jo har row ko uniquely identify karti hai.

• Ye kabhi NULL nahi hoti.

• Ye unique honi chahiye.

 Example:

Student Table

 Yahan par StudentID primary key hai, kyunki ye har student ko unique tarike se pehchaan raha hai.

2. Candidate Key

• Wo sabhi columns jo kisi record ko uniquely identify kar sakte hain.

• Inme se ek ko primary key banaya jata hai.

 Example:

Employee Table

 EmpID, Email, aur Phone sabhi unique hain → sabhi candidate keys hain.

3. Alternate Key

• Candidate keys me se jo primary key nahi banti, wo alternate key hoti hai.

 Example:

Upar wale example me agar EmpID ko primary key banaya jaye, to Email aur Phone alternate keys ban jati hain.

4. Foreign Key

• Foreign key ek aisi key hoti hai jo ek table ke primary key ko dusre table me refer karti hai.

• Iska use tables ke beech connection banane ke liye hota hai.

 Example:

Department Table

Employee Table

 Yahan Employee Table mein DeptID foreign key hai jo Department Table ke DeptID ko refer kar rahi hai.

5. Super Key

• Koi bhi column ya columns ka group jo record ko uniquely identify kare → Super Key

• Sabhi candidate keys super keys hoti hain, lekin sabhi super keys candidate nahi hoti.

 Example:

Super Keys:

• {EmpID}

• {Email}

• {EmpID, Name} → Super key hai par minimal nahi, isliye candidate nahi.

6. Composite Key

• Jab 2 ya 2 se zyada columns milke primary key bante hain, to usse composite key kehte hain.

 Example:

Enrollment Table

 Yahan StudentID + CourseID milkar ek unique record identify karte hain → Composite Key

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 Summary Table