From Zero to Joining 4 Tables: My SQL Learning Path (With Practice Exercises)

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Week 1: Master the Single Table First

Here's what nobody tells beginners: You can't join tables until you truly understand one table.

I spent my first week getting comfortable with basic queries on a single table. This felt slow at first—I wanted to jump straight to the "cool" multi-table stuff—but this foundation made everything else possible.

What I practiced:

Key concepts I learned:

Tables are just rows and columns (like spreadsheets)

Primary keys uniquely identify each record

WHERE clauses filter data

Basic aggregations: COUNT(), SUM(), AVG()

My first practice challenge:

Find all customers who signed up in 2025 and sort them by signup date. This simple exercise taught me to combine WHERE, ORDER BY, and date filtering.

By the end of week one, I could confidently query any single table. That confidence was crucial for what came next.

Week 2, Days 1-3: My First Join (Two Tables)

This was my breakthrough moment.

I finally understood why data splits across multiple tables: it eliminates redundancy. Instead of repeating customer information in every order record, we store it once in a customers table and reference it with a customer_id.

My "aha" moment came when I drew this on paper:

The customer_id is the "bridge" connecting these tables. That's what the JOIN clause does—it matches records by a shared field.

My first real join query:

Three mistakes I made (so you don't have to):

Forgot the ON clause → Got a cartesian product where every customer matched with every order! My 100 customers and 500 orders became 50,000 rows.

Ambiguous column names → Both tables had "customer_id" and SQL didn't know which one I meant. Always use table aliases!

Used INNER JOIN everywhere → Realized later I needed LEFT JOIN to include customers without orders.

The fix: Always use table aliases (c, o) and qualify ambiguous columns (c.customer_id, not just customer_id).

Practice challenge that made it stick:

Join customers and orders, then calculate total spending per customer. This forced me to combine JOINs with GROUP BY and aggregate functions.

By day three, two-table joins felt natural.

Week 2, Days 4-7: Adding the Third Table

Once I understood two-table joins, three tables was just "one more step in the chain."

The real-world scenario: I needed to see what products each customer bought. That required three tables:

customers → orders → order_items

The pattern:

How I understood it:

Think in steps:

Customer → Order (first JOIN)

Order → Items (second JOIN)

Each JOIN adds one table to the chain. The key is following the foreign key relationships from one table to the next.

When it got complex:

Adding WHERE clauses and aggregations:

I learned that:

WHERE filters before grouping

HAVING filters after grouping

GROUP BY requires all non-aggregated columns

The practice challenge:

Find customers who bought products in the 'Electronics' category. This actually required a fourth table (products) to get the category field. That was my bridge to week three.

Week 3: The Four-Table Join

By week three, I was ready for the real challenge: queries that intimidated me three weeks ago.

The business question: "Which customers from New York bought electronics products with a discount in January 2025?"

My approach:

List all needed tables: customers, orders, order_items, products

Map the relationships: customers → orders → order_items → products

Write the query step by step

The full query:

My strategy:

Start with the base table (customers)

Add each join one at a time

Test after each join to catch errors early

Add filters last

Performance lesson I learned the hard way:

This query was slow on 10,000 records until I added indexes:

After indexing: 8 seconds became 0.02 seconds. Always index your foreign keys.

The LEFT JOIN Discovery

Week three also taught me when INNER JOIN isn't enough.

The problem: My manager asked, "Show me ALL customers, including those who haven't ordered."

My INNER JOIN query only showed customers with orders. The others vanished.

The solution:

Key difference:

INNER JOIN: Only matching rows from both tables

LEFT JOIN: All rows from left table + matching rows from right (NULLs for non-matches)

The killer pattern:

Find customers who never ordered:

This pattern (LEFT JOIN + WHERE IS NULL) became my secret weapon for finding missing data.

My 3-Week Practice Schedule

Here's exactly how I structured my practice:

Week 1: Single Table (20 queries)

Basic SELECT, WHERE, ORDER BY

Aggregations: COUNT, SUM, AVG, GROUP BY

Goal: Query any single table confidently

Week 2: Two & Three Tables (30 queries total)

Days 1-3: Two-table INNER JOINs (15 queries)

Days 4-7: Three-table joins (15 queries)

Goal: Understand join relationships

Week 3: Four+ Tables & Complex Queries (15 queries)

Four-table joins

LEFT JOIN scenarios

Performance optimization

Goal: Write any join query needed

Total: 50 queries in 3 weeks. That practice made the difference.

What I Wish I Knew Earlier

Three weeks ago, multi-table joins felt impossible. Today, I write them without thinking. Here's what made it work:

1. Start Visual

Draw tables and their relationships on paper before writing SQL. Seeing the foreign key connections makes everything clear.

2. Follow Foreign Keys

Each join connects through a related ID field. Find those relationships first, then write the ON clause.

3. Build Incrementally

Add one join at a time and test. Don't try to write the entire four-table query at once.

The practice made it stick. Reading about joins helped. Writing 50 queries actually taught me.

Your Next Steps

Ready to start your own journey? Here's what to do:

Get a practice database (SQLite or use online playgrounds like db-fiddle.com)

Follow this 3-week schedule (50 queries total)

Write every query yourself—no copy-paste

When stuck, draw the table relationships on paper

Celebrate small wins (your first 2-table join is huge!)

Three weeks from now, you'll write queries that intimidate you today. I know because I just did it.

Start with one table. Master it. Then add the next.

You've got this.

If this learning path helped you, give it a clap 👏

Drop a comment: What's your biggest SQL join struggle? I read and respond to every one.

Follow me for more SQL tutorials: subqueries, window functions, and query optimization coming soon.