✅ SQL Constraints 📊🛡️

Constraints are the rules that keep your database clean & accurate.

🔹 1. PRIMARY KEY
➤ Uniquely identifies each row in a table
➤ Cannot be NULL or duplicated

CREATE TABLE users (
  user_id INT PRIMARY KEY,
  name VARCHAR(50)
);

🔹 2. FOREIGN KEY
➤ Links to a primary key in another table
➤ Ensures data consistency across tables

CREATE TABLE orders (
  order_id INT PRIMARY KEY,
  user_id INT,
  FOREIGN KEY (user_id) REFERENCES users(user_id)
);

🔹 3. UNIQUE
➤ Ensures all values in a column are different

CREATE TABLE employees (
  id INT PRIMARY KEY,
  email VARCHAR(100) UNIQUE
);

🔹 4. NOT NULL
➤ Column cannot have NULL (empty) values

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

🔹 5. CHECK
➤ Limits the values that can be entered

CREATE TABLE students (
  id INT PRIMARY KEY,
  age INT CHECK (age >= 18)
);

🔹 6. DEFAULT
➤ Automatically sets a default value

CREATE TABLE orders (
  id INT PRIMARY KEY,
  status VARCHAR(20) DEFAULT 'Pending'
);

🎯 Why Constraints Matter:
✔️ No duplicates
✔️ No missing data
✔️ Valid and consistent values
✔️ Reliable database performance

SQL Roadmap: https://whatsapp.com/channel/0029VanC5rODzgT6TiTGoa1v/1394

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