首先我们建立一张带有逗号分隔的字符串。
CREATE TABLE test(id int(6) NOT NULL AUTO_INCREMENT,PRIMARY KEY (id),pname VARCHAR(20) NOT NULL,pnum VARCHAR(50) NOT NULL);

然后插入带有逗号分隔的测试数据
INSERT INTO test(pname,pnum) VALUES('产品1','1,2,4');
INSERT INTO test(pname,pnum) VALUES('产品2','2,4,7');
INSERT INTO test(pname,pnum) VALUES('产品3','3,4');
INSERT INTO test(pname,pnum) VALUES('产品4','1,7,8,9');
INSERT INTO test(pname,pnum) VALUES('产品5','33,4');

查找pnum字段中包含3或者9的记录
mysql> SELECT * FROM test WHERE find_in_set('3',pnum) OR find_in_set('9',pnum);
+----+-------+---------+
| id | pname | pnum    |
+----+-------+---------+
|  3 | 产品3 | 3,4     |
|  4 | 产品4 | 1,7,8,9 |
+----+-------+---------+
2 rows in set (0.03 sec)


使用正则
mysql> SELECT * FROM test WHERE pnum REGEXP '(3|9)';
+----+-------+---------+
| id | pname | pnum    |
+----+-------+---------+
|  3 | 产品3 | 3,4     |
|  4 | 产品4 | 1,7,8,9 |
|  5 | 产品5 | 33,4    |
+----+-------+---------+
3 rows in set (0.02 sec)
这样会产生多条记录，比如33也被查找出来了，不过MYSQL还可以使用正则，挺有意思的

find_in_set()函数返回的所在的位置，如果不存在就返回0
mysql> SELECT find_in_set('e','h,e,l,l,o');
+------------------------------+
| find_in_set('e','h,e,l,l,o') |
+------------------------------+
|                            2 |
+------------------------------+
1 row in set (0.00 sec)

还可以用来排序，如下;
mysql> SELECT * FROM TEST WHERE id in(4,2,3);
+----+-------+---------+
| id | pname | pnum    |
+----+-------+---------+
|  2 | 产品2 | 2,4,7   |
|  3 | 产品3 | 3,4     |
|  4 | 产品4 | 1,7,8,9 |
+----+-------+---------+
3 rows in set (0.03 sec)

如果想要按照ID为4，2，3这样排序呢？
mysql> SELECT * FROM TEST WHERE id in(4,2,3) ORDER BY find_in_set(id,'4,2,3');
+----+-------+---------+
| id | pname | pnum    |
+----+-------+---------+
|  4 | 产品4 | 1,7,8,9 |
|  2 | 产品2 | 2,4,7   |
|  3 | 产品3 | 3,4     |
+----+-------+---------+
3 rows in set (0.03 sec)