Why should you care?

Marketing wants all customers including those who never ordered — find inactive users for win-back SMS.

See it live — copy this example

Run in MySQL Workbench or the mysql CLI.

SELECT c.full_name, o.order_id, o.total_inr
FROM customers c
LEFT JOIN orders o ON o.customer_id = c.customer_id
WHERE o.order_id IS NULL;

What happened?

• All customers appear on the left.
• WHERE o.order_id IS NULL keeps only customers with no matching order — the “never purchased” cohort.

Practice next

1. Ensure some customers have zero orders.
2. Run query; list names with NULL order columns.
3. Remove WHERE to see all customers with repeated rows per order.
4. Move total_inr filter into ON clause and compare results.
5. Count never-ordered users with SELECT COUNT(*) FROM (...).

Remember

LEFT JOIN preserves left-side rows. IS NULL on right PK finds non-matches. Watch WHERE filters that nullify outer join intent.

DataFlow win-back campaign

CRM exports customers with NULL orders for Diwali coupon blast.

Outcome: Reactivation revenue without spamming active buyers.