All data modeling tools support binary relationships, and most support unary relationships, but only a few support n-ary relationships. The reason is that most DBMSs support binary relationships only. As with entities and relationships, there are attribute types and attribute occurrences. An attribute value is an instance or occurrence of an attribute type. An attribute value is a characteristic of or fact about an entity occurrence.
Attribute values are what data processing is all about.
They form the core of information management and represent the most tangible and least abstract aspects of all data processing. Many data modelers divide the world into data and metadata. Metadata is data about data. These objects are called metadata because they are one step removed from the data. Domains A domain is the set of possible values an attribute type can have.