Alx assignment: add SQL exercise solutions for filtering queries in Northwind database

[RealElvince]

Pull Request Description Summary

This pull request introduces solutions to several SQL query exercises using the Northwind SQLite database. The work focuses on practicing filtering data using WHERE, IN, logical operators (AND/OR), and conditions on numeric and boolean-like fields.

Changes Included

Added SQL queries to retrieve:

Names of customers located in Germany.

Products with a Unit Price greater than 30 and marked as discontinued.

Contact names and phone numbers of customers from USA or Canada.

Orders placed by VINET, QUICK, or SUPRD, shipped via shipper 1 or 2.

Challenge query: Employees with the job title Sales Representative or Sales Manager based in London.

Motivation

This pull request contributes to improving SQL querying proficiency, specifically using filtering, logical operators, and pattern matching techniques in analytics workflows.

Testing / Validation

All queries were executed successfully against the Northwind SQLite database

Query results verified manually to ensure correct filtering logic

Execution environment: Jupyter Notebook using SQL Magic (%sql and %%sql)