相互减去PHP变量

Question

I have a table full of page visits, each of the page visits have a unique ID, A visit ID and a Page Time.

By Database structure is below:

  CREATE TABLE IF NOT EXISTS `pages` (
 `id` int(10) NOT NULL AUTO_INCREMENT,
 `client_id` int(11) NOT NULL,
 `url` text NOT NULL,
 `visit_id` varchar(100) NOT NULL,
 `url1` varchar(255) NOT NULL,
 `page_time` time NOT NULL,
 PRIMARY KEY (`id`)
 ) 

What I am trying to work out is, is there a way to subtract one page time from another to identify how long somebody was on the previous page.

So I have a simple query which groups the results

 SELECT * FROM pages group by visit_id

what i am trying to work out, is, is there a way to subtract one page time variable from another in order of their group?

Any ideas would be appreciated

Answer 1

Use session to store the current time when user visits a page. Then when user goes to another page, subtract last loading time from current time and you'll get how long the user was on the last page.