What is a CDM and LDM?

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When designing a new database, you usually start at the "conceptual" level without having to worry about the details of the physical implementation.

A Conceptual Data Model (CDM) is a high-level overview that shows entities and their relationships.

A Logical Data Model (LDM) represents the logical structure of your database: entities, their attributes and relationships. This is independent of the database management system being used.

A LDM include all data objects needed to store all data with regard to the requirements of your database project. However, the physical implementation (via a (generated) PDM) can add additional tables and columns beyond the entities and attributes in the logical model.

With a LDM you can:

•	create a graphical representation of data objects and their relationships, also called an Entity-Relationship Diagram (ERD)

•	provide a high-level overview of the system you want to built

•	focus on the business requirements that the database needs to solve

•	create a solid foundation in order to generate a physical diagram to any of the supported database systems

Note that "relationships" in a LDM are different from "foreign key constraints" in a PDM (or database) when it comes to their representation. A many-to-many relationship, for example, is a relationship between two entities with no intermediate entity, while in a PDM, such a construct will consist of 3 tables.