[8i] Case statement generates ORA-00932: inconsistent datatypes

Question

Note: I am working with an 8i database (yes, it is quite old), and in this situation, I have to deal with the datatypes (i.e. CHAR), I am given to work with.
I am attempting to calculate the amount of time product waits between manufacturing steps. As I've discovered though, sometimes I get a negative value when subtracting the date the previous step completes from the date the current step starts. As it is generally impossible to start a later step before an earlier step (imagine trying to screw a cap onto a bottle that doesn't have threads cut yet--it just can't happen), what I've found is that sometimes two steps are started on the same day and finished on the same day (though not necessarily the day they started). This situation CAN happen when one person did both steps and logged on to both steps at the same time, rather than logging on to one, then the other. So, what I need to do in these situations is replace the negative number with a zero (I'll treat the later step as having no wait time).
Here is some sample data:
(Note: the real data set is the result of a query, and has around 200K rows and more columns, but this should be representative enough to find a solution that works on my actual application.)
CREATE TABLE     steps
(     item_id          CHAR(25)
,     ord_nbr          CHAR(10)
,     sub_nbr          CHAR(3)
,     step_nbr     CHAR(4)
,     start_date     DATE
,     finish_date     DATE
INSERT INTO steps
VALUES ('A','0000000001','001','0010',TO_DATE('01/01/2011','mm/dd/yyyy'),TO_DATE('01/02/2011','mm/dd/yyyy'));
INSERT INTO steps
VALUES ('A','0000000001','001','0020',TO_DATE('01/01/2011','mm/dd/yyyy'),TO_DATE('01/02/2011','mm/dd/yyyy'));
INSERT INTO steps
VALUES ('A','0000000001','001','0030',TO_DATE('01/05/2011','mm/dd/yyyy'),TO_DATE('01/06/2011','mm/dd/yyyy'));
INSERT INTO steps
VALUES ('A','0000000001','002','0010',TO_DATE('01/01/2011','mm/dd/yyyy'),TO_DATE('01/02/2011','mm/dd/yyyy'));
INSERT INTO steps
VALUES ('A','0000000001','002','0020',TO_DATE('01/04/2011','mm/dd/yyyy'),TO_DATE('01/04/2011','mm/dd/yyyy'));
INSERT INTO steps
VALUES ('A','0000000001','002','0030',TO_DATE('01/06/2011','mm/dd/yyyy'),TO_DATE('01/07/2011','mm/dd/yyyy'));
INSERT INTO steps
VALUES ('B','0000000002','001','0005',TO_DATE('01/10/2011','mm/dd/yyyy'),TO_DATE('01/12/2011','mm/dd/yyyy'));
INSERT INTO steps
VALUES ('B','0000000002','001','0025',TO_DATE('01/10/2011','mm/dd/yyyy'),TO_DATE('01/12/2011','mm/dd/yyyy'));Here is the query I use that returns negative values sometimes:
SELECT     item_id
,     ord_nbr
,     sub_nbr
,     step_nbr
,     start_date - last_step_finished
FROM     (
     SELECT     s.*
     ,     LAG (s.finish_date)     OVER     (
                              PARTITION BY     s.item_id
                              ,          s.ord_nbr
                              ,          s.sub_nbr
                              ORDER BY     s.step_nbr
                              )     AS last_step_finished
     FROM     steps s
Returns:
ITEM_ID                   ORD_NBR    SUB STEP START_DATE-LAST_STEP_FINISHED
A                         0000000001 001 0010
A                         0000000001 001 0020                        -1.000
A                         0000000001 001 0030                         3.000
A                         0000000001 002 0010
A                         0000000001 002 0020                         2.000
A                         0000000001 002 0030                         2.000
B                         0000000002 001 0005
B                         0000000002 001 0025                        -2.000These are the results I want to see:
ITEM_ID                   ORD_NBR    SUB STEP START_DATE-LAST_STEP_FINISHED
A                         0000000001 001 0010
A                         0000000001 001 0020                         0.000
A                         0000000001 001 0030                         3.000
A                         0000000001 002 0010
A                         0000000001 002 0020                         2.000
A                         0000000001 002 0030                         2.000
B                         0000000002 001 0005
B                         0000000002 001 0025                         0.000And this is what I tried to do to get those results (comment notes what line generated the error):
SELECT     item_id
,     ord_nbr
,     sub_nbr
,     step_nbr
,     CASE
          WHEN     start_dt - last_step_finished     < 0
          THEN     0
          ELSE     start_dt - last_step_finished  -- THIS LINE GENERATES THE ORA-00932 ERROR
     END                         AS days_in_queue
FROM     (
     SELECT     s.*
     ,     LAG (s.finish_date)     OVER     ( PARTITION BY  s.item_id
                                ,          s.ord_nbr
                                ,          s.sub_nbr
                                ORDER BY     s.step_nbr
                              )     AS last_step_finished
     FROM     steps s
     );I know I've had inconsistent datatype errors before with case statements in this particular 8i database, but I can't seem to figure out why I'm getting one this time. I think it has something to do with the NULL values that can occur for last_step_finished. Also, if I change the case statement to:
,     CASE
          WHEN     start_dt - last_step_finished     < 0
          THEN     NULL
          ELSE     start_dt - last_step_finished  -- THIS LINE GENERATES THE ORA-00932 ERROR
     END     the query runs just fine. But, I don't want NULL, I want 0. In the next level of this query, I will be taking averages by item_id/step_nbr, and I want the 0's to be included in the average. (NULL values, as far as I can tell, would be excluded. AVG(NULL, 1, 2) =AVG (1,2) = 1.5 NOT AVG(0,1,2) = 1).
Thanks in advance!

Answer 1

Thanks, TO_NUMBER did the trick. Since you didn't state in your post where to use TO_NUMBER, here is my final solution, in case anyone in the future looks through this thread to find an answer to their question:
SELECT     item_id
,     ord_nbr
,     sub_nbr
,     step_nbr
,     CASE
          WHEN     start_dt - last_step_finished     < 0
          THEN     0
          ELSE     TO_NUMBER(start_dt - last_step_finished)
     END                         AS days_in_queue
FROM     (
     SELECT     s.*
     ,     LAG (s.finish_date)     OVER     ( PARTITION BY  s.item_id
                                ,          s.ord_nbr
                                ,          s.sub_nbr
                                ORDER BY     s.step_nbr
                              )     AS last_step_finished
     FROM     steps s
     );Edited by: user11033437 on Jun 27, 2011 11:17 AM
I see you edited your post to add TO_NUMBER to it.