组内的汇总函数Dense_rank

Question

Using Oracle 12C Its Regarding Dense_Rank Within Group Aggregate function, this function takes two columns(salary,commission) existing in (emp)table and provide its rank. The results are little confusing. Rank is showed even though the values are not in database. There are two records with 3000.

Existing values in db for

sal, comm

3000,NULL

3000,50 (updated one of the comm value from NULL to 50 in EMP table)

Non Existing values in db for

sal, comm

3000,0

3000,100

3000,500

Is it giving possible ranks,if so then its giving same rank(3) for comm 100,500 and NULL. And equal rank(2) for comm 0 and 50.

ORDER BY SAL DESC AND COMM ASC.

 select DENSE_rank (3000,null) within group(order by sal desc,comm ) DENSE_NULL,
        DENSE_rank (3000,0) within group(order by sal desc,comm) DENSE_ZERO,   
        DENSE_rank (3000,50) within group(order by sal desc,comm) DENSE_50,
        DENSE_rank (3000,100) within group(order by sal desc,comm) DENSE_100,
        DENSE_rank (3000,500) within group(order by sal desc,comm) DENSE_500
   from emp;

Order by sal desc and comm asc

Here 49 is given x rank

50 is given y rank

51 and NULL are given z rank.

SELECT DENSE_RANK (3000,49)   WITHIN GROUP(ORDER BY sal DESC,comm) DENSE_49,   
       DENSE_RANK (3000,50)   WITHIN GROUP(ORDER BY sal DESC,comm) DENSE_50,
       DENSE_RANK (3000,51)   WITHIN GROUP(ORDER BY sal DESC,comm) DENSE_51,
       DENSE_RANK (3000,null) WITHIN GROUP(ORDER BY sal DESC,comm) DENSE_NULL
  FROM emp;

Order by sal desc and comm desc

Here 49 is given x rank

50 and 51 is given y rank

NULL is given z rank.

SELECT DENSE_RANK (3000,49)   WITHIN GROUP(ORDER BY sal DESC,comm desc) DENSE_49,   
       DENSE_RANK (3000,50)   WITHIN GROUP(ORDER BY sal DESC,comm desc) DENSE_50,
       DENSE_RANK (3000,51)   WITHIN GROUP(ORDER BY sal DESC,comm desc) DENSE_51,
       DENSE_RANK (3000,null) WITHIN GROUP(ORDER BY sal DESC,comm desc) DENSE_NULL
  FROM emp;

Answer 1

It must not be concluded that the expressions assign the same rank because each of your DENSE_RANK should be considered in isolation. So, 0 and 50 have same rank because when compared independently with the entries in the table, they both fall under the same rank .

DENSE_NULL DENSE_ZERO   DENSE_50  DENSE_100  DENSE_500
---------- ---------- ---------- ---------- ----------
        63         63         63         63         63

In case, you want to consider multiple hypothetical values to determine rank of the consolidated values, do a UNION ALL with existing entries of the main table.

with hypoth(sal,comm) AS
(
  select 330,null from dual union all
  select 3000,0   from dual union all
  select 3000,50  from dual union all
  select 3000,500 from dual
)
 select DENSE_rank (3000,null) within group(order by sal desc,comm ) DENSE_NULL,
        DENSE_rank (3000,0) within group(order by sal desc,comm) DENSE_ZERO,   
        DENSE_rank (3000,50) within group(order by sal desc,comm) DENSE_50,
        DENSE_rank (3000,100) within group(order by sal desc,comm) DENSE_100,
        DENSE_rank (3000,500) within group(order by sal desc,comm) DENSE_500
   from ( select  salary as sal,commission_pct as comm from  employees 
          UNION ALL
          select sal,comm  from  hypoth
          );

Which gives

DENSE_NULL DENSE_ZERO   DENSE_50  DENSE_100  DENSE_500
---------- ---------- ---------- ---------- ----------
        66         63         64         65         65