Select latest row for each group from oracle

You can use analytic functions

               rank() over (partition by user_id order by ts desc) rnk
          FROM comments c)
 WHERE rnk = 1

Depending on how you want to handle ties (if there can be two rows with the same user_id and ts), you may want to use the row_number or dense_rank function rather than rank. rank would allow multiple rows to be first if there was a tie. row_number would arbitrarily return one row if there was a tie. dense_rank would behave like rank for the rows that tied for first but would consider the next row to be second rather than third assuming two rows tie for first.

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