SQL code of -904 is unsucessful execution of a DB2 query due to an unavailable resource. The name, type of resource unavailable and the reason is also provided by DB2.

When does Sql code -904 comes?

Sqlcode -904 can come if the sql statement tries to query a resource which is unavailable at the time the query is issued  

How the system behaves for sqlcode of -904?

The statements can not be executed. 

How to solve sql code of -904? ; ; ; ;

You can check the name of the resource and the reason for its unavailability and if the reason is out of your control, check  with system administrator or try to issue the query at a later point in time.