What to do if select list is not in group by clause and contains nonaggregated column

A quick fix of select list is not in group by clause and contains nonaggregated column

 is to use ANY_VALUE(contact_id) as contact_id instead of only contact_id in your SELECT.

For example:

SELECT ANY_VALUE(contact_id) as contact_id, ANY_VALUE(mail_sent) as mail_sent, ANY_VALUE(datetime) AS datetime WHERE ... GROUP BY contact_id ORDER BY datetime DESC

Note that the field you group by (e.g. contact_id) must be selected with ANY_VALUE(contact_id) AS contact_id.

You can achieve the same effect without disabling ONLY_FULL_GROUP_BY by using ANY_VALUE() to refer to the nonaggregated column.

Worth to read:

https://dev.mysql.com/doc/refman/5.7/en/group-by-handling.html