Erd

• Entity


  A person, place, object, event or concept in the user environment about which the organization wishes to maintain data
• Entity Type


  A collection of entities that share common properties or characteristics
• Entity Instance


  A singular occurrence of an entity type
• Strong Entity Type


  • Exists independently of other entity types
  • Always have a unique characteristic called an identifier
• Weak Entity Type


  • Existence depends on some other entity type
  • Has no business meaning in the E-R diagram without the entity on which it depends
  • Does not have its own identifier
  • The entity type on which the weak entity depends is called the "identifying owner"
  • The relationship between a weak entity and its identifying owner is called "identifying relationship"
• Attributes


  Properties or characteristics of an entity type that are of interest to the organization
• Attribute Naming


  • Use an initial capital letter followed by lowercase letters
  • If it consists of two words, use an underscore between the words and start each word with a capital letter
• Simple Attribute


  • An attribute that cannot be broken down into smaller components
• Composite Attribute


  • An attribute that can be broken down into component parts
• Single-Valued Attribute


  • An attribute that has only one value
• Multivalued Attribute


  • An attribute that may take on more than one value for a given entity instance
• Derived Attribute


  • An attribute whose values can be calculated from related attribute values (plus possibly data not in the database)
• Identifier Attribute


  • An attribute that uniquely identifies individual instances of an entity type
• Relationship


  An association among the instances of one or more entity types that is of interest to the organization
• Relationship Type


  A meaningful association between (or among) entity types
• Degree of Relationship


  • The number of entity types that participate in a relationship
  • The three most common are: Unary (Degree 1), Binary (Degree 2), Ternary (Degree 3)
• Unary Relationship


  • A relationship between the instances of a single entity type
• Binary Relationship


  • A relationship between the instances of two entity types
• Ternary Relationship


  • A simultaneous relationship among the instances of three entity types
• Cardinality Constraints


  Specifies the number of instances of one entity that can (or must) be associated with each instance of another entity
• Minimum Cardinality


  • The minimum number of instances of entity B that may be associated with each instance of entity A
• Maximum Cardinality


  • The maximum number of instances of entity B that may be associated with each instance of entity A
• Cardinality Examples


  • Mandatory: PATIENT has PATIENT HISTORY
  • One optional, one mandatory: EMPLOYEE is_assigned_to PROJECT
  • Optional: PERSON Is_married_to PERSON
• A PRODUCT LINE may group any number of PRODUCTs but must group at least one PRODUCT. Each PRODUCT must belong to exactly one PRODUCT LINE.
• CUSTOMERs submit ORDERs for products. A CUSTOMER may submit any number of ORDERs. Each ORDER is submitted by exactly one CUSTOMER.
• A professor teaches zero, one or many classes and a class is taught by one professor.