Derived attribute

How to read it

The dashed outline says the value isn’t kept in the database but calculated when needed. Age is the usual example: it changes every year, while the date of birth it comes from doesn’t. Totals are another: a factory’s throughput can be added up from the throughput of each of its assembly lines.

Also searched as: dashed oval er diagram, derived attribute symbol.

Look-alikes

A plain Attribute has a solid outline, and a Multivalued attribute a double one. A dashed underline under an attribute name is a weak entity’s partial key, a different mark. A dashed line between two UML classes is a dependency.

Differences by textbook

The dashed oval is the Elmasri and Navathe convention, followed in course notes such as Juniata College’s and UMBC’s. Silberschatz, Korth and Sudarshan describe derived attributes as ones that “can be computed from other attributes”, with age given a date of birth as their example, but list attributes inside the entity rectangle instead of drawing ovals.