I have a dataset where each observation represents monthly sales per customer. A customer can have multiple months with positive sales and some months with negative sales. I want to remove all customers with at least one month of negative sales. I’ve tried these two examples, but I still have customers with negative sales. What could be the issue?

Example 1:

SELECT *
FROM table_A a
WHERE a.customer NOT IN (
    SELECT customer
    FROM table_A sub_a
    WHERE sub_a.sales < 0
  )
ORDER BY sales;

Example 2:

SELECT a.*
FROM table_A a
WHERE NOT EXISTS (
  SELECT 1
  FROM table_A sub_a
  WHERE sub_a.customer = a.customer
    AND sub_a.sales < 0
)
ORDER BY a.sales;

Any suggestions?