SQL Constraints

🔹 Common Types of Constraints

• PRIMARY KEY — Uniquely identifies each row. Cannot be NULL.

CREATE TABLE Students (
    student_id INT PRIMARY KEY,
    name VARCHAR(50)
);

• FOREIGN KEY — Creates relationship between two tables; refers to a column in another table.

CREATE TABLE Enrollments (
    id INT PRIMARY KEY,
    student_id INT,
    FOREIGN KEY (student_id) REFERENCES Students(student_id)
);

• UNIQUE — Ensures all values in a column are unique.

CREATE TABLE Users (
    email VARCHAR(100) UNIQUE,
    password VARCHAR(50)
);

• NOT NULL — Column cannot have NULL values.

CREATE TABLE Employees (
    id INT PRIMARY KEY,
    name VARCHAR(50) NOT NULL
);

• CHECK — Ensures values meet a condition.

CREATE TABLE Accounts (
    id INT PRIMARY KEY,
    balance DECIMAL CHECK (balance >= 0)
);

• DEFAULT — Provides a default value if none is specified.

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