How do I do a MySQL JOIN on conditions based on mathematical operations across two tables?

Question

I have two tables in a CRM application that I am trying to build.

"Contacts" Table:

id	Name	ContactFrequency (in days)
1	John	7
2	Pete	30

"Events" Table:

id	Contacts_id	Description	Unix_Timestamp
1	1	Sent John an email	1609667504
2	1	Gave John a call	1609645455
1	2	Sent Pete a letterl	1609666755

The "ContactFrequency" is how often I should call that client to stay in touch. Each entry in Events is a call log with a UNIX timestamp. I want to generate a list of clients who need to be called in the next X number of days (or whose next contacts have already passed and are overdue), such as the following (timestamps are completely arbitrary in my examples):

Client	Next Contact Due on (Timestamp)(Ordered by this column)
Pete	1609645352
John	1609634342

How would I do this with a query? I can't wrap my head around it.

Answer 1

Formally:

SELECT Contacts.Name Client, 
       COALESCE(FROM_UNIXTIME(MAX(Events.Unix_Timestamp)) + INTERVAL Contacts.ContactFrequency DAY, CURRENT_TIMESTAMP) NextContactDueOn
FROM Contacts
LEFT JOIN Events ON Contacts.id = Events.Contacts_id
GROUP BY Client
ORDER BY NextContactDueOn DESC