I' am not sure which database connection you are referring at, or which configurations and where...
But as I dig deeper to this problem, I've found out that the problem was on the Glassfish Resources> JDBC> JDBC Resources> jdbc/__default which point to the "DerbyPool".
I've created Resources> JDBC> Connection Pools> MySqlDs and then point the "jdbc/__default" to "MySqlDs", and voila I'd successfully deployed my EJB with EntityManager injected in my Stateless SessionBean(s).
Now, what are the differences when using "DerbyPool" aside from creating my own (live) connection. and DerbyPool seems not running by default. What about when porting to other Application Server...?
Anyone share some lights pertaining to this topic. Thanks very much!