LEFT JOIN and NULLs

A left join keeps every row from the left table. When the right side has no match, its columns are NULL.

Executive summary

Use LEFT JOIN when the left table is the spine — for example all customers, whether or not they have orders. Unmatched right-hand columns arrive as NULL; filter carefully.

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

NULL traps:

• WHERE o.order_id IS NULL finds left rows with no match (customers without orders).
• WHERE o.amount > 100 after a left join quietly drops non-matches (NULL fails the comparison). Put right-table filters in the ON clause when you need to keep unmatched left rows.

SELECT c.customer_id, o.order_id
FROM customers AS c
LEFT JOIN orders AS o
  ON c.customer_id = o.customer_id
 AND o.status = 'paid';

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Check understanding

What happens to unmatched right-table columns in a LEFT JOIN?

• They become NULL
• They delete the left row
• They duplicate the database

How do you list customers with no orders after a LEFT JOIN to orders?

• Filter WHERE order_id IS NULL
• Use INNER JOIN instead
• Drop the customers table

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1. Return to the SQL course overview — you finished the sample path.
2. Explore more practice via the Learning Map or Skills hub.