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Introduction to Entity-Relationship Diagrams
RISE supports Entity-Relationship Diagrams using the Information Engineering notation (crow’s foot). This notation is suitable for printout of real world models, i.e. large models, because of its space conservative handling of relationships. This article provides a brief introduction to ER and the involved symbols and elements.
In ERD, entities are the carriers of data. An entity may have any number of attributes.
Obviously, entities can be directly mapped onto the relational database concept of tables and columns. It can also be seen as a subset of the object oriented concept of classes and members.
In an entity relationship diagram graphic relations are used to denote how entities are related to one another. In short, the circle means "optional" and the vertical bar means "mandatory". The legs indicates that "many" are allowed. The rules specified by these symbols are applied on the entity closest to symbols. For instance, relation c) below specifies that there's a mandatory Apartment for each Tenant where as the Tenant is optional for an Apartment. The examples a) to g) below represents all possible bilateral relation types.
We start with the one-to-one relations event though these aren't the most common when modeling an information solution.  
A more common set of relations are the one-to-many relations.
Finally, entities may be related using many-to-many relations.
Many-to-Many relations are useful primerely in early stage, analysis, modeling. A many-to-many relation can always be transformed into two one-to-many relations by modeling the relation itself as yet another entity.
This gives you the possibility of adding attributes to the entity describing the relation as well and is usually a more fruitful way to model your information if the purpose is developing, based on the model. A good practice is therefore to always check for many-to-many relations to be expanded into two one-to-many relations.
Related articles:
Read ER for database developers for a description on how an ER diagram is mapped to a Relational DataBase Model (RDBM).