Specifies an alias allowing your program or interactive SQL
    statements to refer to more than one database. Use the same alias
    as specified in the ATTACH statement.

    SQL> ATTACH 'ALIAS db1 FILENAME mf_personnel';
    SQL> ATTACH 'ALIAS db2 FILENAME d1';
    SQL> DECLARE :x CHAR(5);
    SQL> BEGIN ON ALIAS db1
    cont> SELECT EMPLOYEE_ID INTO :x FROM db1.EMPLOYEES
    cont> WHERE EMPLOYEE_ID='00164';
    cont> END;
    SQL> PRINT :x;
     X
     00164