In your case you just need to get MAX(Event.DateTime) as LastDateTime

SELECT Event.RO_No,max(Event.DateTime) as LastDateTime FROM Event GROUP BY Event.RO_No

Take also look at

Including an Aggregated Column's Related Values
Including an Aggregated Column's Related Values - Part 2


>If I have the following SQL, I can group by the related field:
>
>

>SELECT Event.RO_No FROM Event GROUP BY Event.RO_No
>

>
>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.
>
>

>SELECT Event.RO_No,Event.DateTime FROM Event GROUP BY Event.RO_No
>

>
>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?