Key attribute

How to read it

A key is unique to each record, so knowing it picks out exactly one. An entity can have more than one key, and each is underlined: an employee might be identified either by a company ID or by a national identification number. Underlining two separate ovals means two separate keys. A key made of several parts together is drawn as a composite attribute, with the composite underlined and its parts not.

A dashed underline is a different mark: the partial key of a Weak entity, which identifies a record only together with its owner’s key.

Also searched as: key attribute symbol, primary key er diagram, underlined attribute.

Look-alikes

A plain Attribute has no underline. A dashed underline is a weak entity’s partial key, and a dashed oval is a Derived attribute.

Differences by notation

Elmasri and Navathe point out that their ER diagrams underline every key, unlike a relational schema, where only the one chosen primary key is underlined. Silberschatz, Korth and Sudarshan underline the primary key in the list of attributes inside the entity rectangle. Crow’s foot diagrams mark keys in various ways: bold and underlined text, or the letters PK. Richard Barker’s notation puts a hash mark (#) before each attribute that is part of the unique identifier.