How to store linked list type data into MYSQL database?

Question

I have a table in Mysql DB like following

mysql> describe APPTBL;
+-------------+-------------+------+-----+---------+----------------+
| Field       | Type        | Null | Key | Default | Extra          |
+-------------+-------------+------+-----+---------+----------------+
| APP_ID      | int(11)     | NO   | PRI | NULL    | auto_increment |
| CREATE_DATE | date        | YES  |     | NULL    |                |
| CLOSE_DATE  | date        | YES  |     | NULL    |                |
| ACTION      | text        | YES  |     | NULL    |                |
+-------------+-------------+------+-----+---------+----------------+

While Starting of App, i am inserting the app details with CREATE_DATE and when it is closing, i am updating its CLOSE_DATE. But in case if user is restarting the app, i want to update NULL in place of CLOSE_DATE column. Here is my query,

mysql> update APPTBL set CLOSE_DATE = NULL where APP_ID = 1;
Query OK, 0 rows affected (0.00 sec)
Rows matched: 1  Changed: 0  Warnings: 0
mysql> select * from APPTBL ;
+-----------+-------------+------------+------------+
|   APP_ID  | CREATE_DATE | CLOSE_DATE |   ACTION   |
+-----------+-------------+------------+------------+
|    1      |  2013-10-08 | NULL       | XXXXXYYYYY |
+-----------+-------------+------------+------------+
1 row in set (0.00 sec)

But why i am not getting any result in this??

mysql> select * from APPTBL where CLOSE_DATE = NULL;
Empty set (0.00 sec)

Also, as i am writhing a java program using this where

rs = st.executeQuery("update APPTBL set CLOSE_DATE = NULL where APP_ID = 1");
if(!rs.wasNull()){
   // Why it is coming here??
}