SQL allows you to specify an alias that declares the database as
    the default database. Specifying a default database means that
    subsequent statements during the database attachment that refer
    to the default database need not use an alias.

    In the SQL module language, the alias specified in the module
    header is the one that designates the default database. In
    precompiled SQL programs and in interactive SQL, the special
    alias RDB$DBHANDLE designates the default database. In all
    environments, omitting an explicit alias is the same as
    specifying the alias that designates the default database.

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    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.