ENTITY RELATIONSHIP DIAGRAM (ERD)

• ENTITY RELATIONSHIP DIAGRAM (ERD)

  -         a high-level data model.

  -         used to define the data elements and relationships for a specified system.

  -         Develops a conceptual design for the database.

  -         Very simple and easy to design view of data

  -         Database structure is portrayed as a diagram called an entity relationship diagram
• 1.      ENTITY

  -         any object, class, person or place.

  -         represented as rectangles.
• A. STRONG ENTITY

  - Will always have a primary key (unique data)

  - Represented by a single rectangle
• B.WEAK ENTITY

  -         depends on another entity.

  -         Doesn’t contain any key attribute of its own.

  -         Represented by a double rectangle
• ATTRIBUTE

  -         Used to describe the property of entity.

  -         ellipse represent an attribute.
• a.    KEY ATTRIBUTE

  -         Used to represent the main characteristics of an entity.

  -         Represents a primary key

  -         Represented by an ellipse with the text underlined.
• b. COMPOSITE ATTRIBUTE

  • Composed of many other attributes.
  • Represented by an ellipse, and those ellipses. are connected with an ellipse.
• c.    MULTIVALUED ATTRIBUTE

  -         Can have more than one value.

  -         Represented by double oval.
• d. DERIVED ATTRIBUTE

  -         Can be derived from other attribute

  -         Represented by a dashed ellipse
• RELATIONSHIP

  -         Used to describe the relation between entities.

  -         Represented by a diamond or rhombus.
• TYPES OF RELATIONSHIP

  a.    ONE-TO-ONE RELATIONSHIP (1:1)

  b.    ONE-TO-MANY RELATIONSHIP (1:M)

  c.    MANY-TO-ONE RELATIONSHIP (M:1)

  d.    MANY-TO-MANY RELATIONSHIP (M:M)
• ONE-TO-MANY RELATIONSHIP (1:M)

  -         One instance of the entity on the left, and more than one instance of an entity on the right associates with the relationship.
• ONE-TO-ONE RELATIONSHIP (1:1)

  -         Only one instance of an entity is associated w/ the relationship.
• MANY-TO-ONE RELATIONSHIP (M:1)

  -         relationship between several instances of one entity and one instance of another entity.
• MANY-TO-MANY RELATIONSHIP (M:M)

  -         relationship is where more than one record in a table is related to more than one record in another table.
• CARDINALITY

  -         Maximum times an entity can relate to an instance w/ another entity or entity set.

  -         Constraints are within the maximum and minimum numbers that are relevant to a relationship.

  -         Represented by lines that have different stylings depending on the type of cardinality that is to be shown.
• ORDINALITY

  -         Minimum time an entity can relate to an instance w/ another entity or entity set.
• GUIDELINES IN NAMING RELATIONSHIPS

  -         r. name is a verb phrase (like Assigned_to, supplies, sends)

  -         represents action being taken, usually in present tense.

  -         avoid vague names (Has or is_related_to)

  -         use descriptive, powerful verb phrases.
• CHARACTERISTICS OF RELATIONSHIPS

  1.      Degree

  2.     Cardinality
• DEGREE OF A RELATIONSHIP

  -         is the number of entity types that participate in it.
• a.    Unary Relationship

  -         When there is an association with only one entity.
• b. Binary Relationship

  -         There is an association among two entities.
• c.    Ternary Relationship

  -         there is an association among three entities