Php Calender Week Random User Calculation

I got an database (phpmyadmin) in which I put users, grid looks like this

id ------ user ------- categorie_bit ------ calender_week

1 ------- Kevin ------ 01------------------- 39

the id and user is self explained. Categorie_bit stands for the current job he / she has to do like having a walk with the dog.

0 - no job

1 - job 1

10 - job 2

11 - both jobs

and the calender week shows the current calender week.

like this in php:

$date = new DateTime();     
echo $date->format('W');

I got various users, each user has to do a job for this calenderweek for this instance 39. The user is picked randomly from the database. (I used a PDO Wrapper)

$table = "users";
$columns = array("id", "firstname");
$orderBy = array('RAND()');

$result_set = $db->select($table,$columns, Null, Null, $orderBy, 1);

In the next step I give the user a "job".

Categorie_bit = 1.

The problem is now that I want to pick a user who did not have a "job" last calender week, so that all users are treated equally.

My first idea is to change the random picker somehow.

Anyone knows how to accomplish this?