🧩 SQL – Phase 1: Foundations

This section covers the fundamental SQL operations used daily by QA Engineers to validate, query, and inspect backend data.
All queries were executed on the qa_testing database using PostgreSQL.

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🎯 Learning Goals

• Retrieve and filter data using SELECT and WHERE
• Sort and limit results with ORDER BY and LIMIT
• Apply text and range filters with LIKE, BETWEEN, and IN
• Combine multiple conditions using AND, OR, and NOT
• Practice real QA-style data checks (email validation, recent orders, product sanity, etc.)

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🧱 Topics Covered

Concept	Description	Example
SELECT	Retrieve data from one or more columns.	SELECT name, city FROM users;
WHERE	Filter rows matching specific conditions.	WHERE city = 'London';
ORDER BY	Sort results ascending or descending.	ORDER BY signup_date DESC;
LIMIT	Restrict the number of rows returned.	LIMIT 5;
LIKE / BETWEEN / IN	Pattern, range, and list filters.	WHERE price BETWEEN 50 AND 200;
AND / OR / NOT	Combine or negate conditions.	WHERE city='London' AND price>100;

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🧩 SQL – Phase 2: Aggregations & Grouping

This section covers the SQL techniques used to summarise data, perform calculations, and generate QA-style analytical reports.
All queries were executed on the qa_testing database using PostgreSQL.

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🎯 Learning Goals

• Count rows and compute totals using COUNT(), SUM(), AVG(), MIN(), MAX()
• Group data using GROUP BY
• Filter aggregated groups using HAVING
• Build conditional summaries using CASE WHEN
• Produce QA reporting outputs (test summaries, orders per user, city distributions)

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🧱 Topics Covered

Concept	Description	Example
COUNT() / SUM() / AVG()	Compute totals and averages.	SELECT COUNT(*) FROM test_results;
MIN() / MAX()	Get smallest / largest values.	SELECT MAX(order_date) FROM orders;
GROUP BY	Aggregate rows into groups.	GROUP BY city
HAVING	Filter groups based on aggregated conditions.	HAVING COUNT(*) >= 2
Conditional Aggregation (CASE)	Create per-status or per-condition summaries.	SUM(CASE WHEN status='passed' THEN 1 END)

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🧱 Tech Stack

• Database: PostgreSQL (qa_testing schema)
• Client: pgAdmin 4 / psql
• Editor: VS Code

To create the schema locally:

-- Run this file in your PostgreSQL instance:
qa_testing_schema.sql