>For the method by program a function, How to do that? Can you show my an example?

PROCEDURE OpenTable
lparameters lcMyTable && parameter is a table
if !empty(gcDatabaseName) && if gcDatabaseName variable contains name of another database than default
  &&gcDatabaseName may contain, for example, "\data\CompanyA\MyDatabase.dbc"
  SET DATABASE TO (gcDatabaseName)
ELSE
  SET DATABASE TO ("\data\MyDatabase.DBC")
endif

&& open table
select 0
use (lcMyTable) again shared

ENDPROC

You may improve it to open table with special alias, buffering etc.
Note that you should define public variable gcDatabaseName in startup program. Than, when you switch to another database, just assign datbase name/path to that variable. In your forms use 'OpenTable("TableName")' in the Load event to open table.

>
>For the method 'SET DEFAULT TO', As the program files and the data files are put at the different directory. Will altering the Default directory cause program error for the exe file unlocated?

No. When you compile application into EXE file, commands like 'do MyProcedure.PRG' or 'DO FORM MyForm.scx' will automatically take program files from EXE file. You do not need to put program files with data, just EXE and data only.

HTH