<P> An entity--relationship model is usually the result of systematic analysis to define and describe what is important to processes in an area of a business . It does not define the business processes; it only presents a business data schema in graphical form . It is usually drawn in a graphical form as boxes (entities) that are connected by lines (relationships) which express the associations and dependencies between entities . An ER model can also be expressed in a verbal form, for example: one building may be divided into zero or more apartments, but one apartment can only be located in one building . </P> <P> Entities may be characterized not only by relationships, but also by additional properties (attributes), which include identifiers called "primary keys". Diagrams created to represent attributes as well as entities and relationships may be called entity - attribute - relationship diagrams, rather than entity--relationship models . </P> <P> An ER model is typically implemented as a database . In a simple relational database implementation, each row of a table represents one instance of an entity type, and each field in a table represents an attribute type . In a relational database a relationship between entities is implemented by storing the primary key of one entity as a pointer or "foreign key" in the table of another entity </P> <P> There is a tradition for ER / data models to be built at two or three levels of abstraction . Note that the conceptual - logical - physical hierarchy below is used in other kinds of specification, and is different from the three schema approach to software engineering . </P>

What type of diagram shows the relationships between tables
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