Functional Dependency in DBMS

Functional Dependency (FD) ek aisa concept hai jo batata hai ki ek column (attribute) ke value se hum doosre column ka value uniquely nikal sakte hain.

 Simple shabdon mein:

Agar aapko ek column ka value pata hai, to aap doosre column ka exact value nikal sakte ho.

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 Formal Definition

Agar relation R me koi attribute Y doosre attribute X par dependent hai, to hum likhte hain:

X → Y
Matlab: X determines Y
(X ke value se Y ka value fix hota hai)

 Example se samjhte hain

Ek Student Table lo:

Yahan pe:

• Roll_No → Name
  (Roll_No se hum Name nikal sakte hain)

• Roll_No → Department
  (Roll_No se hum Department bhi nikal sakte hain)

So yeh dono valid functional dependencies hain.

 Roll_No → Name
 Roll_No → Department

 X Galat Dependency ka Example

Agar same department me multiple students ho:

Yahan Department → Name galat hai, kyunki IT department me multiple students ho sakte hain. Toh:

 Department → Name is not a valid FD

 Functional Dependency ke Types

1. Trivial Functional Dependency

Agar Y, X ka part ho to:

• {Roll_No, Name} → Roll_No

• 
  (Trivial hai, kyunki Roll_No already left side me hai)

2. Non-Trivial Functional Dependency

Agar Y, X ka part nahi ho:

• Roll_No → Name

3. Fully Functional Dependency

Jab Y, X ke poore set par depend kare — na ki uske kisi part pe:

• {Roll_No, Subject} → Marks

• 
  But not just Roll_No → Marks

4. Partial Dependency

Jab Y, X ke kisi part pe depend ho — ye 2NF me remove ki jati hai.

5. Transitive Dependency

Agar:

• X → Y aur Y → Z
  To: X → Z
  (Ye 3NF me remove hoti hai)

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 Functional Dependency ka Importance

Functional Dependency ka use hota hai:

 -> Achhi database design banane me
 -> Repeated data (redundancy) hatane me
 -> Normalization karne me
 -> Data consistency banaye rakhne me

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 Jaldi se Recap: