SQL ranking effective dates

Question

There may be a very simple way to do this, but I can't quite think of it -- I have a dataset that returns a minimum job title and minimum effective date, then all effdts > than the min_effdt. In order to use this data in a charting program, I would like to rank each successive effdt if it exists, as in Min Role Effdt, then 2nd, 3rd, Max. Of course there could be anywhere from 2 to 20 jobs per person.

At first I considered trying a case statement, but I don't think that works when analyzing two columns at once. Is there a SQL statement that will allow ranking? Right now my data looks like

Employee Number | Min Base Role | Min Role Effdt | Base Role | Role Effdt

and comes from two tables, with the 2nd table brought in twice to get the Role / Effdt as Min, then All greater than Min.

I am using ORACLE. Code is below:

 SELECT DISTINCT AL4.FULL_NAME,
 AL4.EMPLOYEE_NUMBER,
            AL4.HIRE_DATE,
            AL4.DATE_OF_BIRTH,
            AL4.AGE,
            AL4.TERM_DATE,
            AL4.ETHNIC_ORIGIN,
            AL2.RECORDVALUE AS MIN_BASE_ROLE,
            AL3.RECORDVALUE AS BASE_ROLE,
            AL3.EFFECTIVE_START_DATE AS "ROLE EFFECTIVE DATE",
            AL2.EFFECTIVE_START_DATE AS "MIN ROLE EFFDT"

  FROM T1 AL2,
   T2  AL3,
   T3  AL4

WHERE AL4.PERSON_ID = AL2.PERSON_ID
AND AL4.PERSON_ID = AL3.PERSON_ID
AND AL4.EMPLOYEE_NUMBER = AL2.HISL_ID
AND AL4.EMPLOYEE_NUMBER = AL3.HISL_ID
AND AL2.RECORDTYPE = 'BASE_ROLE'
AND AL3.RECORDTYPE = 'BASE_ROLE'

AND AL2.EFFECTIVE_START_DATE = (SELECT MIN(A.EFFECTIVE_START_DATE) from T1 A where A.person_id =   al2.person_id and a.recordtype = al2.recordtype)
AND AL3.EFFECTIVE_START_DATE > AL2.EFFECTIVE_START_DATE 
AND (AL4.TERM_DATE >= '01-JAN-2012' or AL4.TERM_DATE is NULL)
order by AL4.EMPLOYEE_NUMBER

Answer 1

The function that you are looking for is row_number() . I think the expression you want is:

row_number() over (partition by AL4.EMPLOYEE_NUMBER
                   order by AL2.EFFECTIVE_START_DATE
                  ) as ranking

The function row_number() says "assign a sequential number to a group of rows". The partition by clause defines the group, where the numbering starts over again at 1 . The order by clause specifies the ordering within the group.

Similar functions rank() and dense_rank() might also be useful. They differ in how they handle duplicate values.