Can you explain the difference between the DENSE_RANK and ROW_NUMBER window functions?

ROW_NUMBER assigns a unique sequential integer to each row within a partition, starting at 1, regardless of duplicate values. It never repeats a number. DENSE_RANK also assigns integers starting at 1, but it gives the same rank to rows with identical values in the ORDER BY clause, and it does not skip subsequent ranks. For example, if a partition has values 10, 10, 20, 30, ROW_NUMBER will produce 1, 2, 3, 4, while DENSE_RANK will produce 1, 1, 2, 3. Use ROW_NUMBER when you need a unique row identifier, such as for pagination. Use DENSE_RANK when you need to group identical values but still want a ranking order. Both functions are O(n log n) in time due to sorting, but the overhead is usually negligible compared to the overall query cost.