Mysql 5.6 - Create view with progressively increment a datetime field grouped by
How to enumerate records in contacts table within contact_group_id groups (emulate ROW_NUMBER) starting with 0:

SELECT t1.contact_id, 
       t1.contact_group_id, 
       t1.contact_email, 
       COUNT(t2.contact_id) - 1 row_number
FROM contacts t1
JOIN contacts t2 ON t1.contact_group_id = t2.contact_group_id
                AND t1.contact_id &gt;= t2.contact_id
GROUP BY t1.contact_id, t1.contact_group_id, t1.contact_email


Use this enumeration in DATEADD().