An entity whose own attributes are insufficient to uniquely identify its instances requires an external identifier and is called a weak entity. For example, a student might be identified by a registration number in combination with their university.
- Connect the weak entity to the identifying entity using a binary relationship.
- Select the weak entity’s participation line and click Ext. Id. Its cardinality automatically becomes (1,1).
- Mark the weak entity’s local identifying attributes, such as
Registration.
The identifying relationship cannot be recursive and cannot contain attributes. Furthermore, an external identifier can involve an entity that is itself weak, as long as no cycle is created.
During translation, the identifying relationship disappears. The weak entity is converted into a relation whose primary key combines the identifiers of all the entities it depends on with its own local identifying attributes.