feat(sql-aggregates): add SQL Aggregated Functions & Set Operations reference

What
- Added `SQL Aggregated Functions & Set Operations.png` as a visual reference.
- Covers key aggregate functions:
  - COUNT() → counts rows.
  - SUM() → totals numeric values.
  - AVG() → calculates average.
  - MIN() / MAX() → find lowest and highest values.
- Includes set operations:
  - UNION → combine results of two queries (distinct).
  - UNION ALL → combine results including duplicates.
  - INTERSECT → return common rows across queries.
  - EXCEPT / MINUS → return rows in first query but not second.

Why
- Aggregate functions are essential for summarizing and analyzing data.
- Set operations allow combining and comparing datasets across queries.
- Visual representation helps learners quickly understand relationships and usage.

How to use
- Use COUNT(), SUM(), AVG(), MIN(), MAX() in SELECT statements with or without GROUP BY.
- Apply set operations when combining results from multiple SELECT queries.
- Leverage this image as a quick reference while writing queries.

Real-life Applications
- Reporting (total sales, average salary, maximum order value).
- Analytics (finding common customers across campaigns using INTERSECT).
- Finance (transactions reconciliation with EXCEPT).
- Data warehousing (merging datasets using UNION/UNION ALL).

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

Author: Someshdiwan