SELECT * FROM table WHERE FIND_IN_SET('2',cid);

------解决方案--------------------

mysql> select find_in_set(2, "2,3,4");
+-------------------------+
| find_in_set(2, "2,3,4") |
+-------------------------+
|                       1 |
+-------------------------+
1 row in set (0.00 sec)

mysql> select find_in_set(2, "52,3,4");
+--------------------------+
| find_in_set(2, "52,3,4") |
+--------------------------+
|                        0 |
+--------------------------+
1 row in set (0.00 sec)

mysql> select find_in_set(2, "2,3,4");  
+-------------------------+
| find_in_set(2, "2,3,4") |
+-------------------------+
|                       1 |
+-------------------------+
1 row in set (0.00 sec)