INNER JOIN

An inner join returns rows where the join keys match in both tables.

Executive summary

Use INNER JOIN when you only want matched pairs — for example orders that have a known customer. Unmatched rows on either side are dropped.

Watch

Video coming soon

Learn

SELECT o.order_id, c.email, o.amount
FROM orders AS o
INNER JOIN customers AS c
  ON o.customer_id = c.customer_id;

Checklist:

• Join on keys that mean the same thing (ids, not free text when you can avoid it).
• Alias tables (o, c) so column names stay readable.
• Watch fan-out: one customer to many orders multiplies rows — that is often correct.

Check

Check understanding

What does INNER JOIN keep?

• Only rows with matching keys on both sides
• All rows from the left table only
• Duplicate tables permanently

Which are good INNER JOIN habits?

Select all that apply

• Join on stable keys
• Use clear table aliases
• Ignore row multiplication from one-to-many keys

What to do next

1. Continue to LEFT JOIN and NULLs.
2. Revisit SELECT, FROM, WHERE if filters feel shaky.