Weak entity

How to read it

A weak entity has no key of its own that singles out each record. Its records are told apart by a partial key together with the key of the entity they depend on, its owner: item 3 of order 1042, say. The relationship that ties a weak entity to its owner is the identifying relationship, drawn as a diamond with a double outline, and the partial key is underlined with a dashed line rather than a solid one.

Peter Chen’s 1976 paper already set the weak entity apart: its example, the dependents of an employee, is “shown as a special rectangular box”, because a dependent is identified through the employee who supports them.

Also searched as: double rectangle er diagram, weak entity symbol.

Look-alikes

A plain Entity has a single outline. On a flowchart, a rectangle with an extra line down each side is a predefined process, not a double rectangle. A double line between a box and a diamond is a different mark again: total participation, meaning every record of that entity takes part in the relationship.

Differences by notation

In Chen’s own terms the weak entity is a “weak entity relation”. The textbooks of Elmasri and Navathe and of Silberschatz, Korth and Sudarshan both draw it as a double rectangle with a double diamond for the identifying relationship. Richard Barker’s notation, used by Oracle’s CASE tools, has no double box: a short line across the relationship near the entity marks the relationship as part of its identifier.