How do I convert a field to non nullable in a query?

SELECT `sales`.`tag_no`, min(`sales`.`branch_code`) AS `first_sold_in180_days`
FROM `bea`.`sales`
WHERE `sales`.`customer_name` != ''
GROUP BY `sales`.`tag_no`
HAVING (min(`sales`.`branch_code`)) > (days_sub(now(), INTERVAL 180 DAY));

+-----------------------+--------------+------+-------+---------+-------+
| Field                 | Type         | Null | Key   | Default | Extra |
+-----------------------+--------------+------+-------+---------+-------+
| tag_no                | varchar(255) | YES  | false | NULL    |       |
| first_sold_in180_days | varchar      | YES  | false | NULL    |       |
+-----------------------+--------------+------+-------+---------+-------+

tag_no

and

first_sold_in180_days

are not nullable by nature and I don't want to deal with Null in the query result(it becomes a materialized view)

would try coalesce like "SELECT coalesce(

sales

.

tag_no

,0), min(coalesce(

sales

.

branch_code

,0)) AS `first_sold_in180_days`"