An alias is a name for a particular attachment to a database.
    Explicitly specifying an alias lets your program or interactive
    SQL statements refer to more than one database.

    Once you specified the alias, you must use it when referring
    to the database in subsequent SQL statements (unless those
    statements are within a CREATE DATABASE statement). You must
    use an alias when you declare more than one database so that SQL
    knows the database to which your statements refer. When you issue
    an ATTACH, CONNECT, CREATE DATABASE, CREATE DOMAIN, CREATE TABLE,
    DECLARE ALIAS, GRANT, GRANT (ANSI-style), IMPORT, REVOKE, or SET
    TRANSACTION statement, you can specify an alias in addition to a
    file specification or a repository path name.

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