Refactor SQL script with primary keys and constraints

cross.sql

@@ -1,31 +1,49 @@
-CREATE TABLE employees (
-    emp_id INT,
-    emp_name VARCHAR(50),
-    dept_id INT
-);
+-- ===============================
+-- CREATE TABLES WITH PRIMARY KEYS
+-- ===============================
+DROP TABLE IF EXISTS employees;
+DROP TABLE IF EXISTS departments;
 
 CREATE TABLE departments (
-    dept_id INT,
-    dept_name VARCHAR(50)
+    dept_id INT PRIMARY KEY,
+    dept_name VARCHAR(50) NOT NULL
 );
 
--- Inserting sample data
-INSERT INTO employees VALUES (1, 'John Doe', 101);
-INSERT INTO employees VALUES (2, 'Jane Smith', 102);
-
-INSERT INTO departments VALUES (101, 'Human Resources');
-INSERT INTO departments VALUES (102, 'Marketing');
-
-
-
-SELECT * from employees;
-SELECT * from departments;
-
--- CROSS JOIN
-SELECT * FROM employees
-CROSS JOIN departments;
-
--- NATURAL JOIN
-SELECT * FROM employees
-NATURAL JOIN departments;
+CREATE TABLE employees (
+    emp_id INT PRIMARY KEY,
+    emp_name VARCHAR(50) NOT NULL,
+    dept_id INT NOT NULL,
+    FOREIGN KEY (dept_id) REFERENCES departments(dept_id)
+);
 
+-- ===============================
+-- INSERT SAMPLE DATA
+-- ===============================
+INSERT INTO departments (dept_id, dept_name) VALUES
+(101, 'Human Resources'),
+(102, 'Marketing');
+
+INSERT INTO employees (emp_id, emp_name, dept_id) VALUES
+(1, 'John Doe', 101),
+(2, 'Jane Smith', 102);
+
+-- ===============================
+-- SIMPLE SELECT QUERIES
+-- ===============================
+SELECT * FROM employees;
+SELECT * FROM departments;
+
+-- ===============================
+-- CROSS JOIN (Cartesian Product)
+-- ===============================
+SELECT e.emp_id, e.emp_name, d.dept_id, d.dept_name
+FROM employees e
+CROSS JOIN departments d;
+
+-- ===============================
+-- INNER JOIN USING dept_id
+-- ===============================
+SELECT e.emp_id, e.emp_name, d.dept_id, d.dept_name
+FROM employees e
+INNER JOIN departments d
+    ON e.dept_id = d.dept_id;