MySQL query random bar record SQL statement and PHP compute probabilities

Recently on the Internet to find the next MySQL query random several sql, I put the final record down.
SELECT * from Uchome_mtag as a joins (SELECT MAX (TAGID) as ID from Uchome_mtag) as B
On (A.tagid>=floor (B.id*rand ())) LIMIT 50
I found a problem after the experiment, when the total number of your table and the number of bars you want to get close, it may not be ideal, it is possible that you have 10, you want to find out the random 8, but only give 5 results. It should be the one that is greater than or equal to the cause.

Also have PHP calculate probability, also organize here.

100% if (true)
if (rand (0,1) = = = 0)
1/3 if (rand (0, 2) = = = 0)
1/x if (rand (0, x-1) = = = 0)
X-if (rand (0, < x)
X‰if (rand (0,999) < x)
/**

* Returns true with the probability of $x/$y
*
* @param int $x
* @param int $y
* @return BOOL
*
* Prob = 1/2
* Prob (1,100) = 1/100
*
*/
function Prob ($x, $y) {
Return (Mt_rand (0, $y-1) < $x);
}