Specifies the name of the database to be accessed in a program
    or by RDO. You must issue an INVOKE DATABASE statement before you
    can use any other statement to reference data in that database.
    Using INVOKE is equivalent to declaring an external subroutine;
    it declares the database to the program.

    Example:

    RDO>  INVOKE DATABASE FILENAME 'DISK2:[DEPT4]PERSONNEL'