Ranking Functions

The ranking functions compute the ordinal rank of a row within the window partition.

These functions can be used with or without partitioning and ordering.However, using them without ordering almost never makes sense.

The ranking functions can be used to create different type of counters.Consider SUM(1) OVER (ORDER BY SALARY) as an example of what they can do, each of them differently.Following is an example query, also comparing with the SUM behavior.

select
    id,
    salary,
    dense_rank() over (order by salary),
    rank() over (order by salary),
    row_number() over (order by salary),
    sum(1) over (order by salary)
  from employee
  order by salary;

Results

id  salary  dense_rank  rank  row_number  sum
--  ------  ----------  ----  ----------  ---
 3    8.00           1     1           1    1
 4    9.00           2     2           2    2
 1   10.00           3     3           3    4
 5   10.00           3     3           4    4
 2   12.00           4     5           5    5

The difference between DENSE_RANK and RANK is that there is a gap related to duplicate rows (relative to the window ordering) only in RANK.DENSE_RANK continues assigning sequential numbers after the duplicate salary.On the other hand, ROW_NUMBER always assigns sequential numbers, even when there are duplicate values.

`CUME_DIST()`

Relative rank (or, cumulative distribution) of a row within a window partition

Result type

DOUBLE PRECISION

Syntax

CUME_DIST () OVER <window_name_or_spec>

CUME_DIST is calculated as the number of rows preceding or peer of the current row divided by the number of rows in the partition.

In other words, CUME_DIST() OVER <window_name_or_spec> is equivalent to COUNT(*) OVER <window_name_or_spec> / COUNT(*) OVER()