How do you find customers who ordered every month in a year?

Short answer: Filter orders to the year, group by customer, and HAVING COUNT(DISTINCT month) = 12.

Sample solution

SELECT CustomerId
FROM Orders
WHERE OrderDate >= '2025-01-01' AND OrderDate < '2026-01-01'
GROUP BY CustomerId
HAVING COUNT(DISTINCT DATEPART(MONTH, OrderDate)) = 12;

COUNT(DISTINCT month) is the key — COUNT(*) alone is wrong if multiple orders/month.