MySQL - Calculate the net time difference between two date-times while excluding breaks?

Question

In a MySQL query I am using the timediff/time_to_sec functions to calculate the total minutes between two date-times.

For example:

2010-03-23 10:00:00
-
2010-03-23 08:00:00
= 120 minutes

What I would like to do is exclude any breaks that occur during the selected time range.

For example:

2010-03-23 10:00:00
-
2010-03-23 08:00:00
-
(break 08:55:00 to 09:10:00)
= 105 minutes

Is there a good method to do this without resorting to a long list of nested IF statements?

UPDATE1:

To clarify - I am trying to calculate how long a user takes to accomplish a given task. If they take a coffee break that time period needs to be excluded. The coffee breaks are a at fixed times.

Answer 1

sum all your breaks that occur during the times, and then subtract to the result of the timediff/time_to_sec function

SELECT TIME_TO_SEC(TIMEDIFF('17:00:00', '09:00:00')) -- 28800

SELECT TIME_TO_SEC(TIMEDIFF('12:30:00', '12:00:00')) -- 1800
SELECT TIME_TO_SEC(TIMEDIFF('10:30:00', '10:15:00')) -- 900

-- 26100

Assuming this structure :

CREATE TABLE work_unit (
 id INT NOT NULL,
 initial_time TIME,
 final_time TIME
)
CREATE TABLE break (
 id INT NOT NULL,
 initial_time TIME,
 final_time TIME
)
INSERT work_unit VALUES (1, '09:00:00', '17:00:00')
INSERT break VALUES (1, '10:00:00', '10:15:00')
INSERT break VALUES (2, '12:00:00', '12:30:00')

You can calculate it with next query:

SELECT *, TIME_TO_SEC(TIMEDIFF(final_time, initial_time)) total_time
, (SELECT SUM(
 TIME_TO_SEC(TIMEDIFF(b.final_time, b.initial_time)))
  FROM break b 
  WHERE (b.initial_time BETWEEN work_unit.initial_time AND work_unit.final_time) OR (b.final_time BETWEEN work_unit.initial_time AND work_unit.final_time)
 ) breaks 
FROM work_unit