One of the most obvious and useful set of window functions are ranking functions where rows from your result set are ranked according to a certain scheme. There are three ranking functions:
ROW_NUMBER()
RANK()
DENSE_RANK()
ROW_NUMBER()
… assigns unique numbers to each row within the PARTITION
given the ORDER BY
clause. So you’d get:
SELECT v, ROW_NUMBER() OVER() FROM t
Note that some SQL dialects (e.g. SQL Server) require an explicit ORDER BY
clause in the OVER()
clause:
SELECT v, ROW_NUMBER() OVER(ORDER BY v)
FROM t
SELECT ROW_NUMBER() OVER(PARTITION BY recovery_model_desc ORDER BY name ASC) AS Row#, name, recovery_model_desc FROM sys.databases WHERE database_id < 5;
The above query returns:
| V | ROW_NUMBER | |---|------------| | a | 1 | | a | 2 | | a | 3 | | b | 4 | | c | 5 | | c | 6 | | d | 7 | | e | 8 |
RANK()
… behaves like ROW_NUMBER()
, except that “equal” rows are ranked the same. If we substitute RANK()
into our previous query:
SELECT v, RANK() OVER(ORDER BY v) FROM t
… then the result we’re getting is this:
| V | RANK | |---|------| | a | 1 | | a | 1 | | a | 1 | | b | 4 | | c | 5 | | c | 5 | | d | 7 | | e | 8 |
As you can see, much like in a sports ranking, we have gaps between the different ranks. We can avoid those gaps by using
DENSE_RANK()
Trivially, DENSE_RANK()
is a rank with no gaps, i.e. it is “dense”. We can write:
SELECT v, DENSE_RANK() OVER(ORDER BY v) FROM t
… to obtain
| V | DENSE_RANK | |---|------------| | a | 1 | | a | 1 | | a | 1 | | b | 2 | | c | 3 | | c | 3 | | d | 4 | | e | 5 |
One interesting aspect of DENSE_RANK() is the fact that it “behaves like” ROW_NUMBER() when we add the DISTINCT keyword.
SELECT DISTINCT v, DENSE_RANK() OVER(ORDER BY v) FROM t
… to obtain
| V | DENSE_RANK | |---|------------| | a | 1 | | b | 2 | | e | 5 | | d | 4 | | c | 3 |
In fact, ROW_NUMBER()
prevents you from using DISTINCT
, because ROW_NUMBER()
generates unique values across the partition before DISTINCT
is applied:
SELECT DISTINCT v, ROW_NUMBER() OVER(ORDER BY v) FROM t ORDER BY 1, 2
DISTINCT
has no effect:
| V | ROW_NUMBER | |---|------------| | a | 1 | | a | 2 | | a | 3 | | b | 4 | | c | 5 | | c | 6 | | d | 7 | | e | 8 |