job for sql, pl/pgsql,gawk,perl or ??

Dear all,

I have this problem with getting information out of my
database and I don't know if it is my knowledge of sql
or that this is something that can't be done in sql.

I have the following table:

id  fdate        prod  price    stat  nr_items  sdate
x1 23-11-2003    123   456      yes     7     01-11-03
x1 23-11-2003    123   456      may     4     07-11-03
x1 23-11-2003    123   400      yes     7     14-11-03
x2 29-11-2003    201   711       no     6     01-11-03
x2 29-11-2003    133   700       no     6     08-11-03

Here, you can see that id x1 is interested in product
123 that we offer for 456 euro's and that he's sure
and wants 7 items. Then almost 6 days afterwards he
mails us to change his order to 4 items and after we
arrange a price reduction.

For various reasons I sometimes want only these
customers and reason as follows:
"Give me the id's of persons wo start with a
status="yes" and end with a status="yes". Then I can
track so called "doubters".

How to do this in postgresql?
So basically I want to be able to scan a file, order
it by id,sdate and the want to look at the field stat.
If I see that the contents of the field changes from
yes to something else and then back to yes, I select
the id. Other positives would be stat started from no
and ending in yes.

The total number of changes an id can have varies.

Can this be done in sql? Or is this a pl/pgsql task?
In that case, what should I think of? Another option
for me is to output a textfile and try to do it in
gawk or perl or something else.




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