PHP/HTML how do you creat a drop down list that open another drop down list?

All, I am writting a php script that runs as part of a joomla site. I have a table

Event

day;

time;

races; (is an array of race id's)

....

here is the Race table

Race

id;

name;

...

On the site I want to have a drop down list of table Event. When they click on an Event in that list, I want to open a second drop down list on that page of possible races (using the values of the Event.race开发者_JAVA百科s for my query). Any idea on how to get the first drop down selected so I can create the second on the page?

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Check out the Son of Suckerfish menu system. This is something you'll need some creative CSS to accomplish, but these guys have already done it and it's pretty easy to implement.

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You can accomplish this with jQuery + an ajax request (aka XMLHttpRequest).