Performing Top-n Analysis (but per group)

Question

The Oracle University Guide SQL Volume 2 says:
To perform Top-n Analysis the general syntax is
SELECT [column_list], ROWNUM
FROM (SELECT [column_list]
FROM table
ORDER BY Top-N_column)
WHERE ROWNUM <= N;
for example
To display the top three earner names and salaries
from the EMPLOYEES table.
SELECT ROWNUM as RANK, last_name, salary
FROM (SELECT last_name,salary FROM employees
ORDER BY salary DESC)
WHERE ROWNUM <= 3;
or to display the four most senior employees in the company.
SELECT ROWNUM as SENIOR,E.last_name, E.hire_date
FROM (SELECT last_name,hire_date FROM employees
ORDER BY hire_date)E
WHERE rownum <= 4;
but what about if I have groups? for example if I want to display the 3 top earners per department?
In my case now
I want to fetch the top 4 items per category, with the biggest quantity (posothta)
SELECT ROWNUM as RANK,
H.KATHG_EIDOYS,
H.KATHG_EIDOYS_DESCR,
H.EIDOS,
H.EIDOS_DESCR,
H.CODE_SUP_BASIKOS_NAME,
H.RAFI_CODE,
H.LINES,
H.POSOTHTA
from (
SELECT B.KATHG_EIDOYS,
D.DESCRIPTION KATHG_EIDOYS_DESCR,
B.CODE EIDOS,
B.DESCRIPTION EIDOS_DESCR,
S.NAME CODE_SUP_BASIKOS_NAME,
C.RAFI_CODE,
COUNT(A.FLD_SEQ_NUM) LINES,
nvl(SUM(decode(k.INV_APOGRAFH_FLAG,'0', decode(k.INV_EXAGOGH_POSOTHTA,1, a.POSOTHTA_TIMOLOGHSHS,2,-a.POSOTHTA_TIMOLOGHSHS))),0) POSOTHTA
FROM ERP_EIDOI_ANA_RAFI C,
ERP_KODIKOI_KINHSHS K,
ERP_POLHSEIS_DETAILS A,
ERP_SUP_CUST S,
ERP_KATHG_EIDON D,
ERP_EIDH b
WHERE B.COMPANY = DECODE(1,1,'9',B.COMPANY)
AND a.COMPANY_KK=K.COMPANY
AND a.KK_CODE=K.CODE
and A.company_WAREHOUSE = c.COMPANY_WARE(+)
and A.MASTER_WAREHOUSE = c.MASTER_WARE_CODE(+)
and A.CODE_WAREHOUSE = c.DETAIL_WARE_CODE(+)
and A.COMPANY_EIDOS = c.COMPANY_EIDOS(+)
and A.EIDOS = c.CODE_EIDOS (+)
AND C.DEFAULT_FLAG (+)= 1
AND b.code = a.EIDOS
and b.company = a.COMPANY_EIDOS
AND D.CODE= B.KATHG_EIDOYS
AND D.COMPANY= B.COMPANY_KATHG_EIDOYS
AND B.COMPANY_SUP_BASIKOS = S.COMPANY
AND B.CODE_SUP_BASIKOS = S.CODE
AND B.PROMHTHEYTHS_FLAG_BASIKOS = S.PELATHS_PROMHTHEYTHS_FLAG
AND /*&p_where*/
a.COMPANY='9' and (a.group_source) = '10' and (A.MASTER_WAREHOUSE) = '01' and (A.CODE_WAREHOUSE) = '0101' and (a.hmerom_parast) >= to_date('01/01/2006','dd/mm/rrrr') and (a.hmerom_parast) <= to_date('25/05/2006','dd/mm/rrrr')
GROUP BY B.KATHG_EIDOYS, D.DESCRIPTION, B.CODE, B.DESCRIPTION, S.NAME,C.RAFI_CODE
ORDER BY 8 DESC
) H
where 1=1 and ROWNUM <= 4
this select does not bring me the desired results, because if for example
category 01 has 10 items
and category 02 has 2 items,
this select will bring me only the first four rows
and not the items from the 02 category.
If you understand what is the case I will wait for your replies.
Thanks in advance

Answer 1

Hi,
Here is an example. It gives you customers ids with highest salary per department.
SELECT CUSTOMER_ID, SALARY, RANK, DEPARTMENT
FROM (SELECT CUSTOMER_ID, SALARY, DEPARTMENT,
RANK() OVER (PARTITION BY DEPARTMENT ORDER BY SALARY DESC) AS RANK
FROM TABLEA)
WHERE RANK < 2;
Peter D.