Syntax
------

DENSE_RANK() OVER (
 [ PARTITION BY partition_expression ]
 [ ORDER BY order_list ]
)

Description
-----------

DENSE_RANK() is a window function that displays the number of a given row,
starting at one and following the ORDER BY sequence of the window function,
with identical values receiving the same result. Unlike the RANK() function,
there are no skipped values if the preceding results are identical. It is also
similar to the ROW_NUMBER() function except that in that function, identical
values will receive a different row number for each result.

Examples
--------

The distinction between DENSE_RANK(), RANK() and ROW_NUMBER():

CREATE TABLE student(course VARCHAR(10), mark int, name varchar(10));

INSERT INTO student VALUES 
 ('Maths', 60, 'Thulile'),
 ('Maths', 60, 'Pritha'),
 ('Maths', 70, 'Voitto'),
 ('Maths', 55, 'Chun'),
 ('Biology', 60, 'Bilal'),
 ('Biology', 70, 'Roger');

SELECT 
 RANK() OVER (PARTITION BY course ORDER BY mark DESC) AS rank,
 DENSE_RANK() OVER (PARTITION BY course ORDER BY mark DESC) AS dense_rank,
 ROW_NUMBER() OVER (PARTITION BY course ORDER BY mark DESC) AS row_num,
 course, mark, name
FROM student ORDER BY course, mark DESC;
+------+------------+---------+---------+------+---------+
| rank | dense_rank | row_num | course  | mark | name    |
+------+------------+---------+---------+------+---------+
|    1 |          1 |       1 | Biology |   70 | Roger   |
|    2 |          2 |       2 | Biology |   60 | Bilal   |
|    1 |          1 |       1 | Maths   |   70 | Voitto  |
|    2 |          2 |       2 | Maths   |   60 | Thulile |
|    2 |          2 |       3 | Maths   |   60 | Pritha  |
|    4 |          3 |       4 | Maths   |   55 | Chun    |
+------+------------+---------+---------+------+---------+

URL: https://mariadb.com/kb/en/dense_rank/