Timestamp calculation

I have three columns on my table, "in", "out" and "totaltime" all of them are timestamp type. So I need to calculate the time a specific person have been logged in the system, and show this result at totaltime column...

this is my code:
String sql = "update school2 set totaltime = TIMESTAMPDIFF(in, out) where id = '"+Integer.parseInt(text_id.getText())+"' ";

it does not work...
I will appreciate any suggestion.

actually, you also have another column in the table, called "id"

you should never store a value in a column like totaltime because it can easily be calculated, so you should remove the totaltime column from your table

in other words, your entire UPDATE statement is not necessary

instead, do the calculation in the SELECT statement when you retireve data for a person

as to why your calculation "doesn't work" i'm afraid i'm not familiar with the "doesn't work" error message

First of all thank you for replay,
I agree with you on the part when you say I should do all my calculation on my select statement, by I will need to keep one record of the total time each person has stood logged on the system, that is why I have created the "totaltime" column.

that's not a good reason, but if you want it that way, go right ahead

I am actually trying to insert the result of the sum of logged time into a new table.