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In the E-R are different elements.

  • An entity, represented by a rectangle. They describe the classes of objects that have common properties.
  • Relations, represented by a diamond. It describes thelogical links between two or more entities. An occurrencereport is an n-tuple consisting of instances of entities. The set of occurrences is a mathematical relationship. You can have relationships and recursive n-ary.
  • Attributes are represented by a dash, and a white cue ball. Describe the properties of entities and relationships.The attributes are all characterized by a domain of existence.

Cardinality represented by two numbers round block. Describe the minimum and maximum values of the attribute associated to the same occurrence.

  • ID of the entity, represented by a lineeta and by a black dot. Describe the concepts of the schema that allow touniquely identify instances of identity. In the case wherethe ID of an entity is obtained by adding or other entitiesusing the ID is said external.

 

  • Generalizations, represented by an arrow. Representlogical links between entities (called father) and one ormore entities (such daughters). The daughters are alsoentities such specializations. In a generalization of each child is an occurrence of the father, in the sense that all property of the father apartiene him to his son. A generalization is total if each occurrence of the occurrenceof a father of daughters entities (otherwise it's part). A generalization is exclusive if it says in each case the parent is at most one occurrence of an entity daughters (ifthey say no overlapping).
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