feat(sql-subquery): add explanation of SQL Subqueries

What
- Introduced documentation on Subqueries (nested queries inside another SQL statement).
- Covered key types:
  - Scalar subquery → returns a single value.
  - Row subquery → returns a single row.
  - Table subquery → returns multiple rows/columns.
- Explained placement of subqueries:
  - In WHERE clause → filtering based on results of another query.
  - In FROM clause → using subquery as a derived table.
  - In SELECT clause → computing values dynamically.
- Provided examples:
  - Find employees earning more than the average salary.
  - List departments with more than 5 employees.
  - Retrieve top 3 highest-paid employees using nested queries.

Why
- Subqueries allow modular, readable, and powerful SQL by embedding logic instead of writing overly complex joins.
- Frequently used in analytics, reporting, and conditional filtering.
- Helps break down complex queries into smaller, more manageable parts.

How to use
- WHERE with subquery:
  ```sql
  SELECT name FROM employees
  WHERE salary > (SELECT AVG(salary) FROM employees);

	•	FROM with subquery:

SELECT dept, COUNT(*)
FROM (SELECT * FROM employees WHERE active=1) AS active_emp
GROUP BY dept;

	•	SELECT with subquery:

SELECT name,
       (SELECT MAX(salary) FROM employees) AS max_salary
FROM employees;

Real-life Applications
	•	HR systems: Find employees above company average salary.
	•	E-commerce: Identify products priced above category average.
	•	Finance: Retrieve customers with account balances higher than the median.
	•	Education: List students scoring above class average.

Notes
	•	Subqueries can often be replaced with JOINs for performance.
	•	Correlated subqueries re-evaluate per row → can be expensive; use carefully.
	•	Aliases are mandatory when subqueries are used in FROM clause.

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

Author: Someshdiwan

Section28JDBCusingSQLite/SQL (Aggregated Functions & Set Operations)/Database Schema.png renamed to Section28JDBCusingSQLite/SQL (Aggregated Functions & Set Operations)/SubQuery SQL/Database Schema.png

@@ -1,5 +1,4 @@
 Definitions:
-
 -  Query: A query is a request to retrieve or manipulate data from a database. It can be a simple or complex
    statement written in SQL.
 
@@ -12,20 +11,21 @@ Definitions:
 
 -  Nested Query: A nested query is another term for a subquery, as it is placed inside another SQL query.
 
-Subqueries are commonly used for filtering, calculations, or comparisons within the main query. 🚀
+Subqueries are commonly used for filtering, calculations, or comparisons within the main query.
 
 
-### Usage of IN, =, and > in SQL  
+### Usage of IN, =, and > in SQL
 
 -  IN → Used when checking if a value exists within a list or the result of a subquery.
    It is useful when expecting multiple possible values.
 
-   Example: Find students who live in any of the given cities.
+Example: Find students who live in any of the given cities.
 
 -  = → Used for exact matches, typically in comparisons with a single value.
    It is used when the subquery or condition returns only one value.
-   Example: "Find students whose department number is exactly the same as Smith’s. 
+
+Example: "Find students whose department number is exactly the same as Smith’s.
 
 -  > → Used for comparing numerical values to check if one is greater than the other.
    It is used when filtering records based on conditions involving numerical magnitude.
-   Example: Find students with marks greater than 50.
+Example: Find students with marks greater than 50.

Section28JDBCusingSQLite/SQL (Aggregated Functions & Set Operations)/SubQuery/Definitions.txt renamed to Section28JDBCusingSQLite/SQL (Aggregated Functions & Set Operations)/SubQuery SQL/Definitions.txt

@@ -6,7 +6,6 @@ SELECT * FROM students
 WHERE city = (SELECT city FROM students WHERE name = 'Smith');
 
 Explanation:
-
 1. The subquery (inner query) executes first: 
 
 SELECT city FROM students WHERE name = 'Smith';
@@ -17,16 +16,16 @@ This retrieves the city where Smith is living (e.g., 'Delhi').
 3. As a result, all students from the same city as 'Smith' will be displayed.
 
 
-### Meaning of the Query:  
+Meaning of the Query:
 
 The query retrieves all students who belong to the same department (deptno) as Smith.  
 
-### Statement: 
- 
+Statement:
+
 Find all students who are in the same department as the student named Smith.  
 
-### Explanation:  
+Explanation:
 
 1. The subquery first finds the deptno of Smith.  
 2. The outer query then selects all students who have the same deptno.  
-3. As a result, all students who share a department with Smith are displayed
+3. As a result, all students who share a department with Smith are displayed