Conceptual and Logical Diagrams

The data models in this chapter allow you to model the semantic and logical structure of your system.

PowerDesigner provides you with a highly flexible environment in which to model your data systems. You can begin with either a CDM (see Conceptual Diagrams) or an LDM (see Logical Diagrams) to analyze your system and then generate a PDM (see the Physical Diagrams) to work out the details of your implementation. Full support for database reverse-engineering allows you to take existing data structures and analyze them at any level of abstraction.

For more information about intermodel generation, see Generating Other Models from a Data Model.

• Supported CDM/LDM Notations
  PowerDesigner supports the most popular data modeling notations in the CDM and LDM. You can choose your notation by clicking ToolsModel Options and selecting it in the Notation list.
• Conceptual Diagrams
  A conceptual data diagram provides a graphical view of the conceptual structure of an information system, and helps you identify the principal entities to be represented, their attributes, and the relationships between them.
• Logical Diagrams
  A logical data diagram provides a graphical view of the structure of an information system, and helps you analyze the structure of your data system through entities and relationships, in which primary identifiers migrate along one-to-many relationships to become foreign identifiers, and many-to-many relationships can be replaced by intermediate entities.
• Data Items (CDM)
  A data item is an elementary piece of information, which represents a fact or a definition in an information system, and which may or may not have any eventual existence as a modeled object.
• Entities (CDM/LDM)
  An entity represents an object about which you want to store information. For example, in a model of a major corporation, the entities created may include Employee and Division.
• Attributes (CDM/LDM)
  In a CDM, attributes are data items attached to an entity, association, or inheritance. In an LDM, there are no data items, and so attributes exist in entities without a conceptual origin.
• Identifiers (CDM/LDM)
  An identifier is one or many entity attributes, whose values uniquely identify each occurrence of the entity.
• Relationships (CDM/LDM)
  A relationship is a link between entities. For example, in a model that manages human resources, the "Member" relationship links the entities Employee and Team and expresses that each employee works in a team, and each team has employees.
• Associations and Association Links (CDM)
  In the Merise modeling methodology an association is used to connect several entities that each represents clearly defined objects, but are linked by an event, which may not be so clearly represented by another entity.
• Inheritances (CDM/LDM)
  An inheritance allows you to define an entity as a special case of a more general entity. The general, or supertype (or parent) entity contains all of the common characteristics, and the subtype (or child) entity contains only the particular characteristics.