You've learned about keys and relationships, but how do you actually answer real business questions that require information from more than one table?

So far, we've used basic SELECT statements and the WHERE clause to filter data, but these tools only work within a single table at a time.

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Do you know why that is?

It's because WHERE doesn't know how the tables are connected.

For example, if your manager asks, "Show me each customer and what they ordered," you'll need to pull data from both the CUSTOMERS and ORDERS tables.

Simply listing both tables in your query isn't enough—the database needs to know how to match up customers with their orders. You need a way to combine related data from different tables based on their relationships.

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Do you know what operation we can use?

This is where JOIN operations become essential. JOINs allow you to combine related tables into a single result set, making it possible to filter and analyze data across multiple tables.

The most common type is INNER JOIN, which returns only the rows with matching values in both tables. Using INNER JOIN, you can answer questions that require information from more than one table.

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Ready to see how it works in practice?

An INNER JOIN finds records that exist in BOTH tables. Think of it as finding the intersection - only rows that have matching connections appear in your results.

If a customer has no orders, they won't appear. If an order has no valid customer, it won't appear either.

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Can you predict what gets excluded with this approach?

Here's the basic INNER JOIN syntax:

The ON clause specifies how tables connect - usually foreign key to primary key.