Use OLAP specifications

Online analytical processing (OLAP) specifications are used to return ranking numbers and row numbers for the result rows of a query. You can specify RANK, DENSE_RANK, and ROW_NUMBER.

Example: Ranking and row numbering

Suppose you want a list of the top 10 salaries along with their ranking. The following query generates the ranking number for you.

 SELECT EMPNO, SALARY 
        RANK() OVER(ORDER BY SALARY DESC),
        DENSE_RANK() OVER(ORDER BY SALARY DESC),
        ROW_NUMBER() OVER(ORDER BY SALARY DESC)  
 FROM EMPLOYEE
 FETCH FIRST 10 ROWS ONLY

This query returns the following information.

Table 1. Results of the previous query
EMPNO	SALARY	RANK	DENSE_RANK	ROW_NUMBER
000010	52,750.00	1	1	1
000110	46,500.00	2	2	2
200010	46,500.00	2	2	3
000020	41,250.00	4	3	4
000050	40,175.00	5	4	5
000030	38,250.00	6	5	6
000070	36,170.00	7	6	7
000060	32,250.00	8	7	8
000220	29,840.00	9	8	9
200220	29,840.00	9	8	10

In this example, the SALARY descending order with the top 10 returned. The RANK column shows the relative ranking of each salary. Notice that there are two rows with the same salary at position 2. Each of those rows is assigned the same rank value. The following row is assigned the value of 4. RANK returns a value for a row that is one more than the total number of rows that precede that row. There are gaps in the numbering sequence whenever there are duplicates.

In contrast, the DENSE_RANK column shows a value of 3 for the row directly after the duplicate rows. DENSE_RANK returns a value for a row that is one more than the number of distinct row values that precede it. There will never be gaps in the numbering sequence.

ROW_NUMBER returns a unique number for each row. For rows that contain duplicate values according to the specified ordering, the assignment of a row number is arbitrary; the row numbers could be assigned in a different order for the duplicate rows when the query is run another time.

Example: Ranking groups

In this example, suppose you want to find out which department has the highest average salary. The following query will group the data by department, determine the average salary for each department, and then rank the resulting averages.

SELECT WORKDEPT, INT(AVG(SALARY)) AS AVERAGE, 
         RANK() OVER(ORDER BY AVG(SALARY) DESC) AS AVG_SALARY 
   FROM EMPLOYEE 
   GROUP BY  WORKDEPT

This query returns the following information.

Table 2. Results of previous query
WORKDEPT	AVERAGE	AVG_SALARY
B01	41,250	1
A00	40,850	2
E01	40,175	3
C01	29,722	4
D21	25,668	5
D11	25,147	6
E21	24,086	7
E11	21,020	8

Example: Ranking within a department

Suppose you want a list of employees along with how their bonus ranks within their department. Using the PARTITION BY clause, you can specify groups that are to be numbered separately.

SELECT LASTNAME, WORKDEPT, BONUS, 
        DENSE_RANK() OVER(PARTITION BY WORKDEPT ORDER BY BONUS DESC) 
          AS BONUS_RANK_IN_DEPT 
   FROM EMPLOYEE
   WHERE WORKDEPT LIKE 'E%'

This query returns the following information.

Table 3. Results of the previous query
LASTNAME	WORKDEPT	BONUS	BONUS_RANK_in_DEPT
GEYER	E01	800.00	1
HENDERSON	E11	600.00	1
SCHNEIDER	E11	500.00	2
SCHWARTZ	E11	500.00	2
SMITH	E11	400.00	3
PARKER	E11	300.00	4
SETRIGHT	E11	300.00	4
SPRINGER	E11	300.00	4
SPENSER	E21	500.00	1
LEE	E21	500.00	1
GOUNOT	E21	500.00	1
WONG	E21	500.00	1
ALONZO	E21	500.00	1
MENTA	E21	400.00	2

Example: Ranking and ordering by table expression results

Suppose you want to find the top five employees whose salaries are the highest along with their department names. The department name is in the department table, so a join is needed. Since ordering is already being done in the nested table expression, that ordering can also be used for determining the ROW_NUMBER value. The ORDER BY ORDER OF table clause is used to do this.

SELECT ROW_NUMBER() OVER(ORDER BY ORDER OF EMP),
             EMPNO, SALARY, DEPTNO, DEPTNAME      
   FROM (SELECT EMPNO, WORKDEPT, SALARY 
             FROM EMPLOYEE        
             ORDER BY SALARY DESC     
             FETCH FIRST 5 ROWS ONLY) EMP,
        DEPARTMENT 
   WHERE DEPTNO = WORKDEPT

This query returns the following information.

Table 4. Results of the previous query
ROW_NUMBER	EMPNO	SALARY	DEPTNO	DEPTNAME
1	000010	52,750.00	A00	SPIFFY COMPUTER SERVICE DIV.
2	000110	46,500.00	A00	SPIFFY COMPUTER SERVICE DIV.
3	200010	46,500.00	A00	SPIFFY COMPUTER SERVICE DIV.
4	000020	41,250.00	B01	PLANNING
5	000050	40,175.00	E01	SUPPORT SERVICES