A relational database diagram depicts the relationships of entity sets stored in a database. An entity in this context is an object, a data component. An entity set comprises entities that are related in some way. Attributes can be assigned to these entities to define their properties. A relationship is simply an association between two or more entities. The relational database diagram primarily uses three basic symbols to represent relationships between elements, entities, and attributes: rectangle, oval, and diamond. Some sub-elements in the diagram are based on the main elements. An entity can be a location, a person, an object, an event, or a concept that stores data in a database. Entities' characteristics must include an attribute and a unique key. Every entity is made up of a set of attributes.