i don't know how to choose a rite one among these three when i write my mapping file.

there are two classes correspondent to two tables. e.g. a user and a login detail. they are one to one relationship, at the same time, loginDetail could be a property or a component of user, what criterians should i consider when i make my decision.

cheers

It is better not have one to one relationships in database for very simple reasons, It needs join to select dependant entities and it needs to maintain integraty. You can avoid it by moving data to single table (the same is about herarchies) It is just optimization, phisical schema doe's not need to look nice, you can make it better with views. Databases can optimize disk usage if you declare nullable fields last, it is not a problem to have more fielts in single table.