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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