JOIN: combining two tables

MySQL Course · lesson 4 of 21 · 5 min read

Rows in one table point at rows in another. JOIN is how you follow the pointer.

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Key points

  • orders.customer_id holds the customers.id it belongs to - a foreign key.
  • A JOIN pairs rows where the ON condition matches, producing one wide row per pair.
  • INNER JOIN keeps only matched pairs, so a customer with no orders disappears.
  • LEFT JOIN keeps every left row, filling the right side with NULL when nothing matched.
  • Aliases (customers c) keep the query readable once two tables are in play.

INNER JOIN answers "who has orders"; LEFT JOIN also answers "who has none" - a different question.

ORDER BY and LIMIT GROUP BY and aggregates

This is a reading copy. The full lesson — with the visual explainer, the interactive lab and a Run button for the code — lives in the MySQL Course course, and every lesson in it is listed on the MySQL Course contents page.