Constraint kya hota hai?

Constraint = database ka rule jo ensure karta hai ki data valid + consistent rahe.

Socho constraint ko “DB ka guard”:

• galat data aane hi nahi deta
• relationships tootne nahi deta
• duplicates rokta
• default fill karta

1) PRIMARY KEY (PK)

PK ka meaning

Primary Key = column(s) jo har row ko uniquely identify kare.

Properties:

• NOT NULL
• UNIQUE
• One PK per table (as a constraint)

Example

CREATE TABLE customer (
  id INT PRIMARY KEY,
  name VARCHAR(100)NOT NULL
);

Composite PK (multi-column PK)

CREATE TABLE enrollment (
  student_id INT,
  course_idI NT,
PRIMARY KEY (student_id, course_id)
);

Must-know points