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Unformatted text preview: t empty. The two boldfaced attributes in the Crow’s Foot notation indicate that the data entry is required. An optional attribute is an attribute that does not require a value, it can be left empty. Domains: A domain is the set of possible values for a given attribute. Attribute may share a domain. The same attribute name is used for different entities, and then they share the same domain. Identifiers or Primary keys: In ERM, an identifier is one or more attributes that uniquely identify each instance or tuple. In the relational model, entities are mapped to tables and the entity identifier is mapped as the table’s primary key (PK). Identifiers and primary keys are underlined in the ERD. Key attributes are also underlined in the relational schema: TABLE_NAME (KEY_ATTRIBUTE1, ATTRIBUTE2, ……, ATTRIBUTE K) Composite Identifiers: Ideally, an entity identifier is only composed of only a single attribute. However, it is possible to have a composite identifier, which is a primary key that is composed of more than one attrib...
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This document was uploaded on 03/07/2014.
- Spring '09