Paramaterised SP returns same results, no matter what parm is used

[SQLWilts]

Hi,
I have MySQL 5 running on a windows box.

I have a weird caching scenario going on here, which I am convinced is the client - but any help pointers would be appreciated.

I have a SP which does a simple select according to parms

CREATE PROCEDURE adcentre.`search_classifiedListings`(
_customerNumber INT,
_directoryId int,
_headingId int,
out _recordCount int)
BEGIN

  select sql_no_cache distinct
    SQL_CALC_FOUND_ROWS
    MAX(l.end_date) as listing_end_date,
    l.tlsearch_listing_code as listing_code,
    l.listing_type_enum,
    l.match_group,
    ld.listing_name as business_name,
    ld.postcode,
    ld.phonenumber as phone_number,
    l.classified_listing_id,
    h.name as heading_name,
    d.name as directory_area
  from ada_classified_listing l
  inner join ada_directory d on d.directory_id = l.directory_id
  inner join ada_heading h on h.heading_id = l.heading_id
  left outer join listing_detail ld on ld.listing_code = l.tlsearch_listing_code
  where
  (l.customer_number is null or l.customer_number = _customerNumber) and
  (_directoryId is null or _directoryId = l.directory_id) and
  (_headingId is null or _headingId = l.heading_id)

  group by directory_area, heading_name, match_group

  order by heading_name, directory_area

  limit 200;

  set _recordCount = found_rows();
END;

If I call the SP like so in MySQL QA I get 8 rows on the first call, and 4 on the second - repeatable.

call adcentre.search_classifiedListings(174605,null,null, @recordCount);

call adcentre.search_classifiedListings(174605,null,208, @recordCount);

If I call them using Toad or my .net app, the first call returns 8 rows, the second 4, if I then use the first call again I only get 4 rows! This will then only return 4 rows until I break and reconnect.

I have turned off query caching on the server, but I am convinced that this is happening on the client somewhere as MySQL QA works fine.

Any thoughts/ideas/contributions appreciated

 

[SQLWilts]

It appears to be the way nulls are being handled.
I am calling the proc as

call myprocname parm1, null, null, null

returns 8 rows

call myprocname parm1, null, null, 208

returns 4 rows

call myprocname parm1, null, null, null

returns the same 4 rows as above, but if I use

call myprocname parm1, null, null, 683

then I get a different result

I am saying

where
  (l.customer_number is null or l.customer_number = _customerNumber) and

Is this correct?