时间计算Timeformat减去

I have an code that shows how much hours sombody worked on that specific day.

SELECT ur.id,us.username, '' as callback_del,datum, ur.description,
        DATE_FORMAT(ur.time_from, '%H:%i') as nosearch_tijdvan,
        DATE_FORMAT(ur.time_till, '%H:%i') as nosearch_tijdtot,
        DATE_FORMAT(break, '%H:%i') as nosearch_pauze,
        TIME_FORMAT(TIMEDIFF(time_from,time_till), '%H:%i') as nosearch_diff,   
        TIME_FORMAT(TIMEDIFF(time_from,time_till)  - break, '%H:%i') AS nosearch_totaal
        FROM " . uren . " AS ur
        JOIN " . project . " AS pr ON pr.project_id = ur.project_id
        JOIN " . user . " AS us ON us.userid = ur.userid

For example

time_from = 07:00:00

time_till = 16:00:00

break = 01:00:00 hour

the output = 08:00:00 hours worked

but if the break = 00:30:00 hours

The output = NULL

Does sombody have any solution for the problem

Thanks

Try

TIMEDIFF(TIMEDIFF(time_from,time_till), break)

instead of

(TIMEDIFF(time_from,time_till)  - break)