How do I get data from my database into my Custom Export Field

I am trying to get data from my database to export to a XML or CSV File by using a custom query. This is my function with the query :

?php
function get_workhours($ID){
    $sql = $wpdb-prepare("SELECT worked_hours FROM db_info WHERE job_id = $ID");
    $data = $wpdb-query($sql);
    if(empty($data)){
        return "0";
    }else{
        return $data;
} ?

And in my custom export field i have this :

[get_workhours{ID}]

And when I try to preview this it gives me the error :

An unknown error occured.

Suggestions for a better idea for this problem are very welcome aswell.

Thanks in advance,

EDIT : I am using the WP All Export Pro Plugin. When adding a field to export I choose the option : Custom export field.