create tabel

1. Create a table called students where each student has an id, name, and age, and ensure that the id uniquely identifies each student.

CREATE TABLE students (
    id INT PRIMARY KEY,
    name VARCHAR(100),
    age INT
);

1. Create a table employees where name and email cannot be empty, but phone_number can be optional.

CREATE TABLE employees (
    id INT PRIMARY KEY,
    name VARCHAR(100) NOT NULL,
    email VARCHAR(100) NOT NULL,
    phone_number VARCHAR(15)
);

1. Create a table users where both username and email must be unique across all records.

CREATE TABLE users (
    id INT PRIMARY KEY,
    username VARCHAR(50) UNIQUE,
    email VARCHAR(100) UNIQUE
);

1. Create a table products where price must always be greater than 0 and stock cannot be negative.

CREATE TABLE products (
    id INT PRIMARY KEY,
    name VARCHAR(100),
    price DECIMAL(10,2) CHECK (price > 0),
    stock INT CHECK (stock >= 0)
);

1. Create a table orders where status should default to 'pending' if no value is provided, and created_at should store the current timestamp automatically.

CREATE TABLE orders (
    id INT PRIMARY KEY,
    status VARCHAR(20) DEFAULT 'pending',
    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
);

1. Create a table accounts where account_number must be unique and not null, and balance must always be greater than or equal to 0.

CREATE TABLE accounts (
    id INT PRIMARY KEY,
    account_number VARCHAR(20) UNIQUE NOT NULL,
    balance DECIMAL(10,2) CHECK (balance >= 0)
);

1. Create a table enrollments where a student can enroll in multiple courses, but the combination of student_id and course_id must be unique.

CREATE TABLE enrollments (
    student_id INT,
    course_id INT,
    PRIMARY KEY (student_id, course_id)
);

1. Create two tables: departments with id and name, and employees with id, name, and department_id, ensuring that department_id in employees must exist in departments.

CREATE TABLE departments (
    id INT PRIMARY KEY,
    name VARCHAR(100)
);

CREATE TABLE employees (
    id INT PRIMARY KEY,
    name VARCHAR(100),
    department_id INT,
    FOREIGN KEY (department_id) REFERENCES departments(id)
);

1. Modify the previous foreign key example so that when a department is deleted, all related employees are also deleted, and when a department ID is updated, it reflects in the employees table.

CREATE TABLE departments (
    id INT PRIMARY KEY,
    name VARCHAR(100)
);

CREATE TABLE employees (
    id INT PRIMARY KEY,
    name VARCHAR(100),
    department_id INT,
    FOREIGN KEY (department_id)
        REFERENCES departments(id)
        ON DELETE CASCADE
        ON UPDATE CASCADE
);