*DEFAULT-DATABASE* — The default database object to use.Variable
Any function or macro in CLSQL that operates on a
      database uses the value of this variable as the default value
      for it's database parameter.
The value of this parameter is changed by calls to
      connect, which sets
      *default-database* to the database object
      it returns.  It is also changed by calls to
      disconnect, when the database object
      being disconnected is the same as the value of
      *default-database*.  In this case
      disconnect sets
      *default-database* to the first database
      that remains in the list of active databases as returned by
      connected-databases, or
      NIL if no further active databases
      exist.
The user may change *default-database* at any time to a valid value of his choice.
If the value of *default-database* is
        NIL, then all calls to CLSQL functions on
        databases must provide a suitable
        database parameter, or an error will be
        signalled.
        (connected-databases)
        => NIL
        (connect '("dent" "newesim" "dent" "dent") :database-type :mysql)
        => #<CLSQL-MYSQL:MYSQL-DATABASE {48385F55}>
        (connect '(nil "template1" "dent" nil) :database-type :postgresql)
        => #<CLSQL-POSTGRESQL:POSTGRESQL-DATABASE {483868FD}>
        (connect '("dent" "newesim" "dent" "dent") :database-type :mysql :if-exists :new)
        => #<CLSQL-MYSQL:MYSQL-DATABASE {48387265}>
        *default-database*
        => #<CLSQL-MYSQL:MYSQL-DATABASE {48387265}>
        (disconnect)
        => T
        *default-database*
        => #<CLSQL-POSTGRESQL:POSTGRESQL-DATABASE {483868FD}>
        (disconnect)
        => T
        *default-database*
        => #<CLSQL-MYSQL:MYSQL-DATABASE {48385F55}>
        (disconnect)
        => T
        *default-database*
        => NIL
        (connected-databases)
        => NIL
      This variable is intended to facilitate working with
        CLSQL in an interactive
        fashion at the top-level loop, and because of this,
        connect and
        disconnect provide some fairly
        complex  behaviour to keep
        *default-database* set to  useful values.
        Programmatic use of CLSQL
        should never depend on the value of
        *default-database* and should provide
        correct database objects via the
        database parameter to functions
        called.