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[[Image:Erd-id-as-primary-key.png|frame|[[Primary key]]]]
[[Image:ER_Diagram_MMORPG.png|thumb|A sample ER diagram]]
An [[entity]] represents a discrete object. Entities can be thought of (roughly) as [[noun]]s. Examples: a computer, an employee, a song, a mathematical theorem. A [[relationship]] captures how two or more entities are related to one another. Relationships can be thought of (again, roughly) as [[verb]]s. Examples: an ''owns'' relation between a company and a computer, a ''supervises'' relation between an employee and a department, a ''performs'' relation between an artist and a song, a ''proved'' relation between a mathematician and a theorem. Entities are drawn as rectangles, relationships as diamonds.
Entities and relationships can both have attributes. Examples: an employee entity might have a [[social security number]] attribute (in the US); the ''proved'' relation may have a ''date'' attribute. Attributes are drawn as ovals connected to their owning entity sets by a line.
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