Coding in WP?

Hi,
I am new to WordPress and need to write PHP-code that queries a table. This table is located in the same database as the WP-tables.
However, if I write the code using the editor in HTML-mode (this is going to be a page), the page preview shows the PHP-code instead of running it.

The code itself is tested and works on another webpage I made up.
Doesn’t WP recognize the “<? PHP ?>”-tag or what am I missing?

Any help would be appreciated,
Erhard