feat(sql-dml): add overview of DML (Data Manipulation Language) commands

What
- Documented key DML commands in SQL that allow manipulation of data within tables:
  - INSERT → add new records into a table.
  - UPDATE → modify existing records.
  - DELETE → remove records.
  - MERGE (in some DBs) → combine insert, update, and delete logic.
- Explained syntax and purpose of each command with examples.
- Clarified difference between DML and DDL (DML changes data, not structure).

Why
- DML is the core of interacting with relational databases — almost every application uses these commands.
- Understanding DML is crucial for building CRUD operations in Java + JDBC.
- Ensures learners know how to manipulate existing data as opposed to just creating schema.

How to use
- INSERT: Add a new employee into `employees` table.
- UPDATE: Change salary for employees in a given department.
- DELETE: Remove inactive customers.
- MERGE: Synchronize staging tables with production tables.

Real-life Applications
- Banking: Update balances after transactions.
- E-commerce: Insert new orders and delete expired carts.
- HR systems: Update employee details (promotions, transfers).
- Analytics: Merge incremental data loads into main warehouse tables.

Notes
- Always use WHERE clause in UPDATE and DELETE to avoid unintentional full-table changes.
- Transactions (COMMIT/ROLLBACK) should wrap DML in critical systems to ensure data integrity.

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

Author: Someshdiwan