How can you find customers who have data for all 12 months in a year using SQL?

You can group the sales data by customer_id and count the distinct months in the target year. A customer qualifies if that count equals 12. Example:

SELECT customer_id

FROM sales

WHERE YEAR(sale_date) = 2023

GROUP BY customer_id

HAVING COUNT(DISTINCT MONTH(sale_date)) = 12;

This query filters to the year, groups by customer, and keeps only those with a distinct month count of 12, meaning they have at least one sale in every month of 2023.