Simple way to calculate median with MySQL

What's the simplest (and hopefully not too slow) way to calculate the median with MySQL? I've used `AVG(x)` for finding the mean, but I'm having a hard time finding a simple way of calculating the median. For now, I'm returning all the rows to PHP, doing a sort, and then picking the middle row, but surely there must be some simple way of doing it in a single MySQL query. Example data: id | val -------- 1 4 2 7 3 2 4 2 5 9 6 8 7 3 Sorting on `val` gives `2 2 3 4 7 8 9`, so the median should be `4`, versus `SELECT AVG(val)` which == `5`.