Conflict in search option

In the advanced search option I put firstname and lastname as separat search options.

But when I try to search on firstname the return is empty.

When I leave the lastname field out, I get returns when I search on firstname.

Why is that ?

the search sql query is build up as this  'AND' that instead of this 'OR' that.
But there could be a other reason why your search isn't working.(misspelled or upper/lower case text issues)

does it make sense?

I want to add a few more search items and broaden the search.

In the search sql query how can I change this  'AND' that to this 'OR' that to broaden the search?

Thanks,

Dan