E-R Diagram & Relational Model

E-R (Entity-Relationship) Diagram ek 'Naksha' hai. Yeh database banane se pehle ka blueprint hai, jisse hum system ke data ko design karte hain. Isse humein pata chalta hai ki system mein kaun-kaun si 'cheezein' (Entities) hain, unki kya-kya properties (Attributes) hain, aur wo aapas mein kaise judi hui hain (Relationships).

A. Core Components of an E-R Diagram

• Entity [Rectangle]: Koi bhi real-world object (e.g., Student, Car). Weak Entity [[Double Rectangle]] apni pehchaan ke liye doosre par depend karti hai.
• Attribute (Oval): Entity ki properties (e.g., Name, Age). Iske types hain: Simple, Composite, Multi-valued ((Double Oval)), Derived (Dashed Oval), aur Key (Underlined).
• Relationship : Entities ke beech ka connection (e.g., Student enrolls in Course).

B. Cardinality Constraints

Yeh batata hai ki ek entity ke kitne instance dusri entity ke kitne instance se jud sakte hain. Yeh 4 tarah ke hote hain:

• One-to-One (1:1): Ek Country ka ek hi President hota hai.
• One-to-Many (1:N): Ek Professor bahut saare Courses padha sakta hai.
• Many-to-One (N:1): Bahut saare Students ek Course me enroll hote hain.
• Many-to-Many (M:N): Ek Student bahut saare Courses le sakta hai, aur ek Course me bahut saare Students ho sakte hain.

C. Mapping to Relational Model (Tables)

E-R diagram se actual tables banane ke rules. Sabse important rule hai ki Many-to-Many (M:N) relationship ke liye hamesha ek naya, alag table banta hai, jisme dono entities ke Primary Keys as Foreign Keys aate hain.