Attributes or Properties are characteristics of an entity. In the example in Figure 2 attribute names are shown as a combination of upper and lower case characters inside bubbles. Multivalued attributes are often shown in a diagram by the use of a douple circle as is done in figure 2. Multivalued attributes may have multiple values for an individual occurrence of an entity. A Candidate Key is an attirubte or set of attributes that uniquely identify an instance of an entity.
Some entities have more than one candidate key.
Sometimes it is necessary to combine attributes to form a composite key. Primary Key Attribute. A primary key is usually a single attribute, but may be a comprised of a concatenation of attributes. The term identifier is also used for primary key. The primary key selection must follow certain criteria.