Dependency Preserving Decomposition in DBMS 

Jab hum kisi relation (yaani table) ko normalization ke process mein chhoti-chhoti tables mein todte hain, toh humein ye ensure karna hota hai ki koi bhi important rule (Functional Dependency) loss na ho.

 Dependency Preserving Decomposition ka matlab:

Jab hum table ko todte hain, toh jo functional dependencies (FDs) pehle wali table mein thi, wo sab new tables mein bhi apply ho paayein — bina join kiye.

 Kyun zaroori hai?

Agar decomposition dependency preserve nahi karti, toh kuch rules tab tak apply nahi ho paate jab tak aap tables ko dobara join na karo. Ye slow aur inefficient hota hai.

 Example se Samjhte Hain:

 Original Table: Student

Functional Dependencies (FDs):

1. RollNo → Name, Dept

2. Dept → HOD

 Ab Decomposition karo:

R1(RollNo, Name, Dept)
R2(Dept, HOD)

Check karo:

• RollNo → Name, Dept R1 mein hai →  Preserved

• Dept → HOD R2 mein hai →  Preserved

 Dono dependencies new tables mein exist karti hain.
 Isliye, Dependency Preserving Decomposition hai.

 Jab Dependency Preserve Nahi Hoti:

Original Table:
R(A, B, C)

FDs:

1. A → B

2. B → C

Decomposition:
R1(A, B)
R2(A, C)

Check karo:

• A → B  R1 mein hai 

• But B → C kisi bhi table mein nahi hai 

 B → C ko preserve karne ke liye R1 aur R2 ko join karna padega.
 Isliye ye dependency preserving nahi hai.

 Dependency Preserve karne ka fayda:

• -> Functional rules ko asaani se apply kar sakte ho

• -> Joins ki zarurat nahi padti

• -> Database design efficient aur clean banta hai

 Summary Table