I want to do something like this:

select MAX(field1), field2 from tbl1 group by field1

But the above query as is doesn't work (sqlplus throws an error). How can I achieve above in a single query (for now, I have split it into two queries to get the result).

You could use an inline view and an analytic function (max() over()) in it for selecting the row(s) with the largest timestamp:

select field1, field2 from
    (select field1, field2, max(field1) over() as max_field1
     from tbl1)
where field1 = max_field1;

Note that if there are many records with a timestamp value of max_field1, they are all returned (in arbitrary order).

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