Entity
An entity is a collection of distinguishable real world objects with common properties such as student and course.
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- An entity refers to the entity set (a table) and not to a single entity occurrence (a row).
- An entity is represented by a rectangle ▭ containing the entity’s name.
An attribute is a data item that describes a property of an entity or a relationship like
fname and cno.
- Attributes have a domain. The domain is the attribute’s set of possible values.
- Each entity has an identifier (a set of attributes or candidate keys). Each entity has a primary identifier (primary key).
- Descriptor is an attribute, but not identifier.
- Composite attribute is an attribute that can be further subdivided to yield additional attributes.
- Simple attribute is an attribute cannot be subdivided.
- Single-valued attribute can have only a single value.
- Multi-valued attribute can have many values (double lines).
- Attributes are represented by ovals ⬭ that are connected to the entity by a line. Each oval contains the name of the attribute it represents.
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Never in the history of calming down has anyone calmed downed by being told to calm down. |