When you design an OO class-diagram, realize that you are building a functional view of the system. Objects are not merely inanimate things: they are moving parts.
A database schema built to support an OO-designed system should be thought of as a way to provide "persistence" to those objects .. as well as a "huge filing cabinet of reference information that can be used to create new instances of objects."
The architecture of a typical (SQL) database may or may not be object-oriented. Nonetheless, one of the characteistics of your OO design must be that "only the objects know" about the database. You deal with the objects; the objects deal with the DB.