Access :: Modify Select Statement To Show All Jobs In Database

My students are working on an ASP.net VB application for a Web Programming competition and we are having problems with a page where we want an URL string to return all values from our database. Here is the page: [URL] If you use the jobs by category menu on the left side, if you select a job category (ie retail) it will pull up all the jobs from that category. By using a query in the URL like this: [URL] What we need is an option that would pull up all of the records regardless of category. So we made a menu option that passes the value "all". Like this: [URL] We use this information for the dataset we created (we're using Dreamweaver, ASP.net VB and and Access Dbase). We made a dataset SQL as follows:

SELECT *
FROM jobs, businesses
WHERE jobs.contactlogin=businesses.contactlogin and jobcategory = ?
ORDER BY jobs.dateposted DESC

This works fine if a value is passed. For example if "retail" is picked from the menu it passes the value just fine and returns all jobs that have "retail" as the job category in the database. My problem is, how do I modify the above Select statement to show all jobs in the database if the value "all" is passed for the jobcategory. My first thought was to create an If statement that says something to the effect of:

If jobcategory = all THEN
SELECT *
FROM jobs, businesses
WHERE jobs.contactlogin=businesses.contactlogin
ORDER BY jobs.dateposted DESC
ELSE
SELECT *
FROM jobs, businesses
WHERE jobs.contactlogin=businesses.contactlogin and jobcategory = ?
ORDER BY jobs.dateposted DESC
END IF

But I haven't been able to figure out the correct syntax. I also tried making this work by using a query in MS Access:

SELECT *
FROM jobs, businesses
WHERE jobs.contactlogin=businesses.contactlogin
AND (jobcategory = @jobcategory or @jobcategory = 'all')
ORDER BY jobs.dateposted DESC;

(which does work within MS Access) and calling the query in the Dreamweaver dataset but apparently MS Access can't do that.This seems like it should be easy but so far this problem is kicking my butt.