Your First Join: INNER JOIN

The Matching Game

The sales team needed a report that combined order amounts with customer names. The data lived in two separate tables. It was time for my first real join.

The Quest: The Intersection

An `INNER JOIN` returns only rows that have a match in **both** tables. If an order has a `customer_id` that doesn't exist in the `customers` table, that order will be excluded from the results.

Think of it as the "Intersection" of a Venn Diagram.

The Implementation: The Syntax

SELECT

o.order_id,

o.order_amount,

c.name,

c.email

FROM orders o

INNER JOIN customers c ON o.customer_id = c.id;

Breaking It Down

1. **FROM orders o**: Start with the `orders` table, alias it as `o`.

2. **INNER JOIN customers c**: Bring in the `customers` table, alias it as `c`.

3. **ON o.customer_id = c.id**: This is the "Key" that links them. Match rows where these two columns are equal.

The "Oops" Moment

I once got back fewer rows than I expected. Some orders had a NULL `customer_id` (guest checkouts). Since NULL doesn't match anything, those rows were silently dropped.

**Pro Tip**: Always compare your row count before and after a join. If it drops, investigate why. Are there orphan keys or NULLs?

The Victory

The sales report now showed "John Doe: $500, Jane Smith: $1,200..." instead of just "Customer 101: $500." The data finally made sense to a human.

Your Task for Today

Write an `INNER JOIN` between two tables in your database. Compare the row count of the joined table to the original source table.

*Day 33: Preserving Data—LEFT JOIN.*