It's not clear what DateTime value you need. The latest one?

SELECT Event.RO_No, MAX(Event.DateTime) AS LatestDateTime FROM Event GROUP BY Event.RO_No

>If I have the following SQL, I can group by the related field:
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>SELECT Event.RO_No FROM Event GROUP BY Event.RO_No
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>Now, I need to add a secondary field in regards to the grouping. Basically, RO_No is a foreign key. So, there are multiple occurences of it in the Event table. So, in the result, I need to grab a datetime field. If I do this, it will not work.
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>SELECT Event.RO_No,Event.DateTime FROM Event GROUP BY Event.RO_No
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>The reason is that Event.DateTime is not included in the group. But, in this case, I cannot include it in the group as it will take all the records from the table. As RO_No is not my primary key and there are multiple occurences of it in the table, I cannot use the first SQL and subset that into another SQL to get the related additional needed fields. How to adjust that to fit the requirements?