see fulltext searching at mysql.com.

from http://dev.mysql.com/doc/refman/5.1/en/fulltext-search.html
.....
 The following example is more complex. The query returns the relevance
values and it also sorts the rows in order of decreasing relevance. To
achieve this result, you should specify MATCH() twice: once in the
SELECT list and once in the WHERE clause. This causes no additional
overhead, because the MySQL optimizer notices that the two MATCH()
calls are identical and invokes the full-text search code only once.

mysql> SELECT id, body, MATCH (title,body) AGAINST
    -> ('Security implications of running MySQL as root') AS score
    -> FROM articles WHERE MATCH (title,body) AGAINST
    -> ('Security implications of running MySQL as root');
+----+-------------------------------------
+-----------------+
| id | body                                                       |
score           |
+----+-------------------------------------
+-----------------+
|  4 | 1. Never run mysqld as root. 2. ...             |
1.5219271183014 |
|  6 | When configured properly, MySQL ...        | 1.3114095926285 |
+----+-------------------------------------
+-----------------+
2 rows in set (0.00 sec)
.....

http://dev.mysql.com/doc/refman/5.1/en/fulltext-search.html