How to read only the most current records in PL/SQL

Question

Oracle version: 11.2
I have a table in ORACLE called RATES and there is a EFFECTIVE_DATE, RATE_NAME, RATE_AMOUNT
RATE
1 RATE_ONE 1/1/2000 0.10
2 RATE_ONE 5/1/2005 0.15
3 RATE_ONE 12/1/2010 0.20
4 RATE_TWO 10/15/2009 0.33
5 RATE_THREE 9/7/2011 0.05
6 RATE_THREE 12/31/2011 0.06
A rate can be in there multiple times, but the most recent rate is the active rate. I need to query this table and only want to bring back the currently active rows:
RECORDS 3, 4 and 6
How can I do this in the most elegant way?
Can I say:
SELECT RATE WHERE trunc(sysdate) between EFFECTIVE_DATE and trunc(sysdate);
(this will not work. I'm not sure how to do this without an exp date column and then I could say
SELECT RATE WHERE trunc(sysdate) between EFFECTIVE_DATE and END_DATE
The problem is that I cannot add that column at this time, so need to find a way to do it programatically
Edited by: userLynx on Feb 1, 2012 8:56 AM

Answer 1

Try This:
WITH a
        AS (SELECT 'RATE_ONE' ratename,
                   TO_DATE ('1/1/2000', 'mm/dd/yyyy') eff_date,
                   0.10 amount
              FROM DUAL
            UNION ALL
            SELECT 'RATE_ONE' ratename,
                   TO_DATE ('5/1/2005', 'mm/dd/yyyy') eff_date,
                   0.15 amount
              FROM DUAL
            UNION ALL
            SELECT 'RATE_ONE' ratename,
                   TO_DATE ('12/1/2010', 'mm/dd/yyyy') eff_date,
                   0.20 amount
              FROM DUAL
            UNION ALL
            SELECT 'RATE_TWO' ratename,
                   TO_DATE ('10/15/2009', 'mm/dd/yyyy') eff_date,
                   0.33 amount
              FROM DUAL
            UNION ALL
            SELECT 'RATE_THREE' ratename,
                   TO_DATE ('9/7/2011', 'mm/dd/yyyy') eff_date,
                   0.05 amount
              FROM DUAL
            UNION ALL
            SELECT 'RATE_THREE' ratename,
                   TO_DATE ('12/31/2011', 'mm/dd/yyyy') eff_date,
                   0.06 amount
              FROM DUAL)
SELECT ratename, eff_date, amount
  FROM (SELECT ratename,
               eff_date,
               amount,
               RANK () OVER (PARTITION BY ratename ORDER BY eff_date DESC) rk
          FROM a)
WHERE rk = 1