A Dense Rank In Oracle

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DENSE_RANK computes the rank of a row in an ordered group of rows and returns the rank as a NUMBER. The ranks are consecutive integers beginning with 1. The largest rank value is the number of unique values returned by the query. Rank values are not skipped in the event of ties. Rows with equal values for the ranking criteria receive the same rank. This function is useful for top-N and bottom-N reporting.
This function accepts as arguments any numeric datatype and returns NUMBER.
As an aggregate function, DENSE_RANK calculates the dense rank of a hypothetical row identified by the arguments of the function with respect to a given sort specification. The arguments of the function must all evaluate to constant expressions within each aggregate group, because they identify a single row within each group. The constant argument expressions and the expressions in the order_by_clause of the aggregate match by position. Therefore, the number of arguments must be the same and types must be compatible.
As an analytic function, DENSE_RANK computes the rank of each row returned from a query with respect to the other rows, based on the values of the value_exprs in the order_by_clause.

Aggregate Example:

SELECT DENSE_RANK(15500, .05) WITHIN GROUP 
   (ORDER BY salary DESC, commission_pct) "Dense Rank" 
   FROM employees;

         Dense Rank
-------------------
                  3

Analytic Example:

SELECT d.department_name, e.last_name, e.salary, DENSE_RANK() 
   OVER (PARTITION BY e.department_id ORDER BY e.salary) AS drank
   FROM employees e, departments d
   WHERE e.department_id = d.department_id
   AND d.department_id IN ('30', '40');

DEPARTMENT_NAME         LAST_NAME              SALARY      DRANK
----------------------- ------------------ ---------- ----------
Purchasing              Colmenares               2500          1
Purchasing              Himuro                   2600          2
Purchasing              Tobias                   2800          3
Purchasing              Baida                    2900          4
Purchasing              Khoo                     3100          5
Purchasing              Raphaely                11000          6
Human Resources         Marvis                   6500