#1055 - Expression #1 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'db.w.user_id' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by
SELECT * FROM discount
LEFT JOIN company ON discount.company_id=company.id
WHERE TIMESTAMPDIFF(DAY,end_date,NOW()) = 0
ORDER BY percent_discount DESC
LIMIT 1