DataBase System Mathematical relations g Each attribute of a relation has a name g The set of allowed values for each attribute is called the domain(tax) of the attribute Attribute values are(normally required to be atomic; that is indivisible Note: multivalued attribute(多值属性) values are not atomic H Note: composite attribute(组合属性) values are not atomic G The special value null is a member of every domain E The null value causes complications in the definition of many operations Haichang Gao, Software School, Xidian University 6DataBase System Haichang Gao , Software School , Xidian University 6 Mathematical Relations  Each attribute of a relation has a name  The set of allowed values for each attribute is called the domain（域） of the attribute  Attribute values are (normally) required to be atomic; that is, indivisible  Note: multivalued attribute(多值属性) values are not atomic  Note: composite attribute (组合属性) values are not atomic  The special value null is a member of every domain  The null value causes complications in the definition of many operations