feat(sql-joins): add explanation of Cartesian Product in SQL

What
- Documented the concept of Cartesian Product (CROSS JOIN) in SQL.
- Explained how it occurs when no join condition is provided.
- Illustrated with example tables (`Employees`, `Departments`) and resulting rows.
- Highlighted row count calculation: (rows in Table A × rows in Table B).
- Covered both intentional and unintentional use cases.

Why
- Helps differentiate between meaningful joins and accidental cross joins.
- Clarifies how missing `ON` conditions can lead to unexpected results.
- Provides awareness for developers working with SQL joins.

Key Points
- CROSS JOIN or missing ON condition → Cartesian Product.
- Useful for generating all possible combinations (e.g., customers × products).
- Avoid unintentional Cartesian Products by using:
  - INNER JOIN
  - LEFT JOIN
  - RIGHT JOIN
  with explicit `ON` conditions.

Real-life Applications
- Generating test datasets by pairing rows across tables.
- Recommendation engines (all users × all items).
- Scheduling systems (all employees × all time slots).

Signed-off-by: https://github.com/Someshdiwan <[email protected]>

Author: Someshdiwan

Section28JDBCusingSQLite/JoinsInSQL/Cartesian Product in SQL Joins.txt

@@ -4,11 +4,11 @@ A Cartesian Product occurs in SQL when two tables are combined without specifyin
 
 It results in every row from the first table being paired with every row from the second table.
 
-How It Happens
+How It Happens:
 - If you use a CROSS JOIN explicitly or an incorrect JOIN without an ON condition, you get a Cartesian Product.
 - The total number of rows in the result set = (Rows in Table A) × (Rows in Table B).
 
-Example Scenario
+Example Scenario:
 
 Tables:
 1. Employees Table:
@@ -44,15 +44,14 @@ If no join condition is used, each employee is combined with all departments:
    | 2      | Bob   | IT       |
    | 2      | Bob   | Sales    |
    +--------+------+-----------+
-   ```
+
 
 Here, 2 employees × 3 departments = 6 rows.
 
 When Cartesian Products Are Useful:
 - When you intentionally want all possible combinations (e.g., pairing all customers with all products).
 - If using CROSS JOIN explicitly.
 
-How to Avoid Unwanted Cartesian Products
-
+How to Avoid Unwanted Cartesian Products:
 - Always specify a JOIN condition with ON to ensure meaningful relationships between tables.
-- Use INNER JOIN, LEFT JOIN, or RIGHT JOIN when needed.
+- Use INNER JOIN, LEFT JOIN, or RIGHT JOIN when needed.