PostgreSQL-constraints

Constraints: Constraints are the backbone of reliable database design. They act like safety checks, ensuring that every piece of data stored is valid, consistent, and meaningful. Types of Constraints 1.PRIMARY KEY -Ensures each row has a unique identifier. -No duplicates, no NULL values. example: emp_id SERIAL PRIMARY KEY 2.FOREIGN KEY -Creates a relationship between two tables. -Ensures values in one table match values in another. example: dept_id INT REFERENCES Department(dept_id) 3.UNIQUE -Guarantees that all values in a column are distinct. example: email VARCHAR(150) UNIQUE 4. NOT NULL -Prevents empty values in a column. example: name VARCHAR(100) NOT NULL 5.DEFAULT -Assigns an automatic value if none is provided during insertion. example: status VARCHAR(20) DEFAULT 'Active' 6. CHECK -Validates data against a condition. example: salary NUMERIC(10,2) CHECK (salary > 0) Example table: CREATE TABLE Employee ( emp_id SERIAL PRIMARY KEY, name VARCHAR(100) NOT NULL, email VARCHAR(150