NULL value

i use heidisql with xampp on my pc and i have a database in mysql 4.1 on a linux server when i use a query whith " where column is NULL " its work on linux but on my local test with xampp the same miror database i have to put " colum is '' " to obtain a result the column field is char 2 for a code like 'HS' 'OK' and so on default value NULL i have search for NULL in preference but not found any idee ? thanks for help

I can't really follow your explanation but I'm pretty sure this has nothing to do with HeidiSQL.

NULL and '' (empty string) are entirely different things in all relational database engines except Oracle (and Oracle is wrong). MySQL Server can, however, be configured to happily accept lots of errors and data loss events. Please Google about MySQL's SQL_MODE variable.