Flattening, correlated subquery

Question

Hi,
The following fields are key values on a table, with example data :
EMPNO DATE ACTION
1234 6/6/05 HIR
1234 6/6/05 XFR
1234 6/10/05 DTA
1234 6/22/05 XFR
1234 7/30/05 TER
3456 6/6/05 HIR
etc.
I need to create a view to show the previous 5 rows of history per employee but flattened to be one line per Employee Number (EMPNO).
So the view would be something like :
EMPNO ACTION1 DATE1 ACTION2 DATE2 ACTION3 DATE3 (etc.)
1234 HIR 6/6/05 XFR 6/6/05 DTA 6/10/05
I have thought of aliasing the table and then using correlated subqueries, but this got complicated joining to >2 tables. Since joining to table 2 required that DATE1 <> DATE2 or ACTION1 <> ACTION2. But then alias 3 becomes more convoluted and then 4 and 5 OTT.
I also looked at some rownum trickery to get the <=5 rows, but I could not get this working.
What is the best solution for this problem ? Note that it does have to be a pure view, not PL/SQL etc.
Thanks,
T0rrent

Answer 1

I think you are after this. You must know the number of actions per output row. In this case five:
drop table t;
create table t( empno number, x_date date, action varchar2(3));
insert into t values (1234, to_date('6/6/05 01:00','mm/dd/yy hh:mi'),'HIR'); --need to have unique date values per empno
insert into t values (1234, to_date('6/6/05 02:00','mm/dd/yy hh:mi'),'XFR'); --need to have unique date values per empno
insert into t values (1234, to_date('6/10/05','mm/dd/yy'),'DTA');
insert into t values (1234, to_date('6/22/05','mm/dd/yy'),'XFR');
insert into t values (1234, to_date('7/30/05','mm/dd/yy'),'TER');
insert into t values (3456, to_date('6/6/05','mm/dd/yy'),'HIR');
commit;
create or replace view tv as
  select empno, x_date, action, dense_rank() over( partition by empno order by x_date ) e_rank
  from t
  order by empno;
SELECT  empno,
        max( decode( e_rank, 1, x_date||' '||action)) e_act1,
        max( decode( e_rank, 2, x_date||' '||action)) e_act2,
        max( decode( e_rank, 3, x_date||' '||action)) e_act3,
        max( decode( e_rank, 4, x_date||' '||action)) e_act4,
        max( decode( e_rank, 5, x_date||' '||action)) e_act5
FROM    tv
group by empno;
     EMPNO E_ACT1        E_ACT2        E_ACT3        E_ACT4        E_ACT5
      1234 06-JUN-05 HIR 06-JUN-05 XFR 10-JUN-05 DTA 22-JUN-05 XFR 30-JUL-05 TER
      3456 06-JUN-05 HIR