Due to impedance mismatch between relational technology and object you need to understand the progress of mapping classes (objects) and their relationships to tables and relationships between them in a database. Classes show both behavior and data whereas relational database tables just insert data. Database schemas have keys whereas object schema does not have keys and instead use instance references to implement relationships to other objects.
1. Classes map to tables in a way but not directly.
2. An attribute of a class may be mapped to zero or more columns in a database. Not all attributes are consistent.
3. Some attributes of an object are objects itself. This is a recursive relationship where at some point the attribute will be mapped to more columns or zero.
4. In its simple form an attribute maps to a single column whereas each has similar type. When you prepare mapping with different kinds then you will use to be able to convert them back and forth.