Use sequence for dynamic conditional grouping

Question

Hi experts,
I have a simple(?) task - I want to number (assign) some groups (increasing group number) based on a condition.
Here an example: create numbered groups based on department number (actual the same grouping)
create table emp as
select * from scott.emp;
CREATE SEQUENCE group_no;
SELECT group_no.NEXTVAL FROM DUAL;
SELECT group_no.CURRVAL FROM DUAL;
/* automatic grouping - NOT WORKING!!!! */
  select ename, deptno, deptno_next, deptno_prev,
   case when (deptno_next > deptno) then group_no.nextval else 1 end grp
    from
      (select ename, deptno,
              lead(deptno) over (order by deptno) deptno_next,
              lag(deptno) over (order by deptno) deptno_prev
      from emp
PROBLEM:
It seems the sequence is increased, even when the condition for a new number is not met!
NAME      DEPTNO                 DEPTNO_NEXT            DEPTNO_PREV            GRP                   
CLARK      10                     10                                            1                     
KING       10                     10                     10                     1                     
MILLER     10                     20                     10                     130                   
JONES      20                     20                     10                     1                     
FORD       20                     20                     20                     1                     
ADAMS      20                     20                     20                     1                     
SMITH      20                     20                     20                     1                     
SCOTT      20                     30                     20                     135                  
WARD       30                     30                     20                     1                     
TURNER     30                     30                     30                     1                     
ALLEN      30                     30                     30                     1                     
JAMES      30                     30                     30                     1                     
BLAKE      30                     30                     30                     1                     
MARTIN     30                                            30                     1instead of else 1 I will use group_no.currval (same group number, when not changing department). I know, I need to cover the previous number as well, but I made it simple, because the problem here seems to be the sequence...
Looking forward to your support :-)
Duik
Edited by: user10939560 on 08.10.2012 07:42
Edited by: user10939560 on 08.10.2012 07:44
Edited by: user10939560 on 08.10.2012 07:45
Edited by: user10939560 on 08.10.2012 07:48
Edited by: user10939560 on 08.10.2012 07:49

Answer 1

Yes, your observation is correct.
You cant use a sequence this way.
May be the below gives you a hint to achieve your goal.
select
ename
,empno
,deptno
,case
when lag(deptno, 1, deptno) over (order by deptno) = deptno
then deptno
else 30*deptno
end mark
from emp
order by deptno
ENAME     EMPNO     DEPTNO     MARK
clark     7782     10     10
miller     7934     10     10
KING     7839     10     10
ford     7902     20     600
scott     7788     20     20
jones     7566     20     20
SMITH     7369     20     20
adams     7876     20     20
WARD     7521     30     900
martin     7654     30     30
...Example with consecutive ordering
select
ename
,empno
,deptno
,sum(grp) over (order by deptno) grp
from (
select
ename
,empno
,deptno
,case
when lag(deptno, 1, deptno) over (order by deptno) = deptno
then 0
else 1
end grp
from emp
order by deptno
ENAME     EMPNO     DEPTNO     GRP
clark     7782     10     0
miller     7934     10     0
KING     7839     10     0
ford     7902     20     1
scott     7788     20     1
jones     7566     20     1
SMITH     7369     20     1
adams     7876     20     1
WARD     7521     30     2
martin     7654     30     2
...Edited by: chris227 on 08.10.2012 08:08
consecutive ordering