Table aliases

Give a table a short name in FROM and JOIN.

Multi-table JOIN

You can join many tables with successive JOINs. Each JOIN gets its own ON condition.

SELECT o.id, c.name, p.name
FROM orders o
JOIN customers c ON o.customer_id = c.id
JOIN products p ON o.product_id = p.id;

This query combines order, customer and product in one query.

o.id, c.name, p.name is selected from orders o. customers c is joined on o.customer_id = c.id, keeping only matching rows. products p is joined on o.product_id = p.id, keeping only matching rows.

Multi-table JOIN — details

Each JOIN merges the previous result with a new table. In multi-table queries aliases are almost mandatory.

Exercise Multi-table JOIN

Join orders, customers and products on customer_id and product_id.

Show example solution

SELECT o.id, c.name, p.name
FROM orders o
JOIN customers c ON o.customer_id = c.id
JOIN products p ON o.product_id = p.id

Test yourself

What does each JOIN need in a multi-join?
- A shared WHERE
- A single FROM
- Its own ON condition
- GROUP BY
Show answer

Its own ON condition — Each JOIN gets its own ON.

Three tables

FROM orders o
JOIN customers c ON o.customer_id = c.id
____ products p ON o.product_id = p.id;

Show answer

FROM orders o
JOIN customers c ON o.customer_id = c.id
JOIN products p ON o.product_id = p.id;

Table aliases: match the terms

JOIN

join
ON

condition
alias

short name
FROM

table

Next lesson: Multi-table JOIN

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