# Entity Relationship (ER) Diagram

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* ER model is the **blueprint** of a database
* Graphical tool for modeling data
* ER Diagrams are translatable into **relational tables**
* **Relationships** between entities can be 1-1 (one to one), 1-N (one to many), or N-M (many to many)
* **Primary Key** identifies an instance of the entity. Unique, never changing, never null.
* **Foreign Key** is used to link two tables together, can be replicated, can be null.