Anti-joins

Study the pattern

A LEFT JOIN with a NULL check finds rows without a related record. Predict the output before running the example.

SELECT c.customer_id,c.name FROM customers c LEFT JOIN orders o ON o.customer_id=c.customer_id WHERE o.order_id IS NULL ORDER BY c.customer_id;

Explain it from memory

What does today's SQL feature do, and when would you use it?

LEFT JOIN orders, then check o.order_id IS NULL.

Run SQL in your browser

Run the example, then modify one clause and predict how the result changes.

LEFT JOIN orders, then check o.order_id IS NULL.

Consolidate the skill

• Run and inspect the output.
• Change one part and predict the difference.
• Return later and rewrite the query from memory.