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.