10.4 Window Frames

A window frame specifies which rows to consider for the current row when evaluating the window function.

The frame comprises three pieces: unit, start bound, and end bound. The unit can be RANGE or ROWS, which defines how the bounds will work.

The bounds are:

• <expr> PRECEDING

• <expr> FOLLOWING

• CURRENT ROW

• With RANGE, the ORDER BY should specify exactly one expression, and that expression should be of a numeric, date, time, or timestamp type. For <expr> PRECEDING, expr is subtracted from the ORDER BY expression, and for <expr> FOLLOWING, expr is added. For CURRENT ROW, the expression is used as-is.

  All rows inside the current partition that are between the bounds are considered part of the resulting window frame.

• With ROWS, ORDER BY expressions are not limited by number or type. For this unit, <expr> PRECEDING and <expr FOLLOWING relate to the row position within the current partition, and not the values of the ordering keys.

Both UNBOUNDED PRECEDING and UNBOUNDED FOLLOWING work identical with RANGE and ROWS. UNBOUNDED PRECEDING start at the first row of the current partition, and UNBOUNDED FOLLOWING the last row of the current partition.

The frame syntax with <window_frame_start> specifies the start-frame, with the end-frame being `CURRENT ROW.

Some window functions discard frames:

• ROW_NUMBER, LAG and LEAD always work as ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW

• DENSE_RANK, RANK, PERCENT_RANK and CUME_DIST always work as RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW

• FIRST_VALUE, LAST_VALUE and NTH_VALUE respect frames, but the RANGE unit behaviour is identical to ROWS.

Example Using Frame

When the ORDER BY clause is used, but a frame clause is omitted, the default considers the partition up to the current row. When combined with SUM, this results in a running total:

select
  id,
  salary,
  sum(salary) over (order by salary) sum_salary
from employee
order by salary;

Result:

| id | salary | sum_salary |
|---:|-------:|-----------:|
|  3 |   8.00 |       8.00 |
|  4 |   9.00 |      17.00 |
|  1 |  10.00 |      37.00 |
|  5 |  10.00 |      37.00 |
|  2 |  12.00 |      49.00 |

On the other hand, if we apply a frame for the entire partition, we get the total for the entire partition.

select
  id,
  salary,
  sum(salary) over (
    order by salary
    ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING
  ) sum_salary
from employee
order by salary;

Result:

| id | salary | sum_salary |
|---:|-------:|-----------:|
|  3 |   8.00 |      49.00 |
|  4 |   9.00 |      49.00 |
|  1 |  10.00 |      49.00 |
|  5 |  10.00 |      49.00 |
|  2 |  12.00 |      49.00 |

This example is just to demonstrate how this works; the result of this specific example would be simpler to produce with just sum(salary) over().

We can use a range frame to compute the count of employees with salaries between (an employee’s salary - 1) and (their salary + 1) with this query:

select
  id,
  salary,
  count(*) over (
    order by salary
    RANGE BETWEEN 1 PRECEDING AND 1 FOLLOWING
  ) range_count
from employee
order by salary;

Result:

| id | salary | range_count |
|---:|-------:|------------:|
|  3 |   8.00 |           2 |
|  4 |   9.00 |           4 |
|  1 |  10.00 |           3 |
|  5 |  10.00 |           3 |
|  2 |  12.00 |           1 |