If you do not declare any alias or attach to any database, SQL
    declares the database specified in the logical name SQL$DATABASE
    or configuration parameter SQL_DATABASE as the default database,
    using the FOR FILENAME clause.

    If you declare a database specifying an alias other than the
    one that declares the default database, you must use that alias
    to qualify names of any database elements to which you refer in
    SQL statements. If you omit the alias, SQL assumes the table or
    view must be part of the default database. If there is no default
    database and you omit the alias, SQL generates an error.