Grouping or Windowing
Many analytic functions also allow you to specify a virtual, moving
window surrounding a row within a partition. You do this using the
framing
clause. Such moving windows are useful for running
calculations such as a running total.
The following, Oracle-based example uses the
framing
clause on the analytic variant of
SUM to calculate a running sum of the values in
the first column. No partitioning clause is used, so each invocation
of SUM operates over the entire result set.
However, the ORDER BY
clause sorts the rows for
SUM in ascending order of NUM’s value, and the
BETWEEN
clause (which is the windowing clause)
causes each invocation of SUM to include values
for NUM only up through the current
row. Each successive invocation of SUM includes
yet another value for NUM, in order,
from the lowest value of NUM to the
greatest:
SELECT NUM, SUM(NUM) OVER (ORDER BY NUM ROWS
BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) S FROM ODD_NUMS;
NUM S --------- ---------- 0 0 1 1 2 3 3 6
This example’s a bit too easy, as the order of the final result set happens to match the order of the running total. That doesn’t need to be the case. The following example generates the same results, but in a different order. You can see that the running total values are appropriate for each value of NUM, but the rows are presented in a different order than before. The result set ordering is completely independent of the ordering used for window function calculations:
SELECT NUM, SUM(NUM) ...
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