从表中选择并正确绑定在DB2 / Oracle列上

Question

I have a scenario where I have to select rows from a table and given the requirements, I cannot come up with a sql statement where it could retrieve the rows correctly.

ID  EFFDT        STATUS
1   2001-01-01   A
1   2002-01-01   A
1   2003-01-01   B
1   2004-01-01   B
1   2005-01-01   A
1   2006-01-01   B

I would like to grab all the rows which were not changed at the point of time and get their mininum effective date.

I tried something like:

Select e.* 
from employee a 
where e.effdt = (Select min(b.effdt) 
                 from employee b 
                  where e.id= b.id 
                 and e.status= b.status)

It returns two rows:

ID  EFFDT       STATUS
1   2001-01-01  A    
1   2003-01-01  B

Expected output :

ID   EFFDT        STATUS
1    2001-01-01   A
1    2003-01-01   B
1    2005-01-01   A
1    2006-01-01   B

I need the minimum effective dated row before a status was changed for the same employee multiple times.

Any help would be really appreciated. Thanks.

Answer 1

Try this:

SELECT id, effdt, status
FROM (
  SELECT e.*,
       CASE WHEN id <> @last_id THEN 1
            WHEN status <> @last_status THEN 1
            ELSE 0
       END take_this_row,
       @last_id := id last_id,
       @last_status := status
  FROM employee e
  cross join (
    SELECT @last_id := -111,
           @last_status := 'XXX'
  ) init_var
  ORDER BY id, effdt
) q
WHERE take_this_row = 1

demo --> http://www.sqlfiddle.com/#!2/dec99c/7

---

EDIT

A query for Oracle:

SELECT id, effdt, status
FROM (
  SELECT e.*,
       CASE lag( status ) over (partition by id order by effdt )
            WHEN status THEN 0
            ELSE 1
       END take_this_row
  FROM employee e
)
WHERE take_this_row = 1
ORDER BY id, effdt

Demo --> http://www.sqlfiddle.com/#!4/1d27f/5

This query will probaby run also on DB2, unfortunately I have no DB2 installed on my PC and cannot test it.
I've found here: https://www.ibm.com/developerworks/community/blogs/dbtrue/entry/lag_lead_first_value_last_value_totally_new_in_db21?lang=en
that in DB2 9.7 they have implemented analytic functions LAG, LEAD etc (in their terminology "OLAP functions"), therefore this query should work on DB2.