Further Learning Into SQL

Introduction

Instead of following textbook examples, today I worked with actual .sql files. This made the SQL feel more alive — more like a tool I could use in the real world rather than something abstract.

This post walks through what I did with the SQL files I used on May 20, and what each file helped me understand better.

---

What I Practiced

📁 File: w3schools

This file contained queries that made me apply and review the following:

• Date filtering using MONTH() and YEAR() functions
• Aggregate functions like COUNT(), AVG() on grouped results
• Grouping data using GROUP BY on customer behavior
• Filtering groups using HAVING instead of WHERE

Example:

SELECT customer_id, COUNT(*) AS order_count
FROM orders
GROUP BY customer_id
HAVING order_count >= 3;

This helped me understand that:

• HAVING works after aggregation
• Without GROUP BY, I can’t use HAVING
• WHERE filters before, HAVING filters after

---

What I Learned

• SQL is not just about selecting rows — it’s about summarizing behavior
• Function order (WHERE, GROUP BY, HAVING, ORDER BY) is critical
• Simple mistakes (like putting HAVING without GROUP BY) gave syntax errors that helped me remember structure better
• Writing a query from scratch and debugging why it fails teaches more than copying one that already works

---

What I Thought About

While working with this file, I realized:

• I’m not afraid of trying longer queries anymore
• It’s better to read and modify existing SQL than always write from zero
• Comments in .sql files are like documentation for your thinking — and I want to start leaving more of them

---

What I Want to Do Next

• Practice writing a summary report from orders (like total revenue per month)
• Try using nested subqueries to filter on aggregated conditions
• Learn how to refactor long queries into readable blocks
• Explore creating views or temporary tables for reuse

---

This session reminded me why I like SQL — it’s not just a language, it’s a way of thinking logically with data.
And every file I work with helps me reason more clearly.