Use NOT IN with a subquery to find exclusions
How many customers received emails but never opened a single one? These are our 'unreachable' segment. I need a count of distinct customers who have sends where opened = 0 but never appear in any send where opened = 1.
Each hint you reveal reduces the XP you can earn. Try the query first.
First find who DID open: `SELECT DISTINCT customer_id FROM fact_sends WHERE opened = 1`
Then find customers with sends who are NOT in that list: `WHERE customer_id NOT IN (...)` and filter to `opened = 0`
Count distinct customers: `SELECT COUNT(DISTINCT customer_id) AS never_opened` — a customer may have multiple unopened sends