In a relational database, a weak entity is an entity that cannot be uniquely identified by its attributes alone; therefore, it must use a foreign key in conjunction with its attributes to create a primary key. The foreign key is typically a primary key of an entity it is related to. Each element in the weak entity set must have a relationship with exactly one element in the owner entity set, and therefore, the relationship cannot be a many-to-many relationship
. Weak entities have total participation constraint (existence dependency) in their identifying relationship with the owner entity
. They are represented with a double rectangle in an ER Diagram, and the identifying relationships are represented with a double diamond
. An example of a weak entity is the "header/detail" records in many real- world situations such as claims, orders, and invoices, where the header captures information common across all forms and the detail captures information specific to individual items
