Attributes in an ER Diagram: The 6 Types Explained

What is an attribute?

An attribute is a property that describes an entity. If an entity is a single student, then that student's Roll_no, Name, Age, and Gender are its attributes — the pieces of information we store about it. In an ER diagram, the entity set is drawn as a rectangle and each of its attributes is drawn as an oval joined to that rectangle by a line.

The six types of attributes

Attributes are grouped by how they behave. There are six common types: simple, composite, single-valued, multi-valued, derived, and key. It helps to know that these describe different properties rather than six separate boxes — so one attribute can be several of them at the same time. Age, for example, is simple, single-valued, and derived all at once. The diagram below shows the notation for each type.

Simple and composite attributes

A simple (or atomic) attribute cannot be divided into smaller parts — Roll_no, Age, and Gender are simple because each is already a single, whole value. A composite attribute can be split into smaller sub-attributes. Name is composite because it divides into First_name, Middle_name, and Last_name, and Address is composite because it divides into Street, City, State, and Country.

• Simple attribute: cannot be divided further — for example, Roll_no, Age, Gender.
• Composite attribute: divides into sub-attributes — for example, Name (First, Middle, Last) or Address (Street, City, State, Country).

Single-valued and multi-valued attributes

A single-valued attribute can hold only one value for a given entity. A student has exactly one Roll_no, one Date of Birth, and one Gender, so these are single-valued. A multi-valued attribute can hold more than one value for the same entity. A student may have several phone numbers or email addresses, so Mobile_no and Email_id are multi-valued. In an ER diagram, a multi-valued attribute is drawn as a double oval.

• Single-valued attribute: takes exactly one value per entity — for example, Roll_no, DOB, Gender.
• Multi-valued attribute: can take more than one value per entity — for example, Mobile_no, Email_id. Drawn as a double oval.

Derived attributes

A derived attribute is one whose value can be worked out from another attribute instead of being stored on its own. Age is the classic example: rather than store a student’s age (which would need updating every year), we store the Date of Birth and compute Age from it whenever we need it. In an ER diagram, a derived attribute is drawn as a dashed oval.

Key attributes

A key attribute is an attribute whose value identifies an entity uniquely within its entity set. No two students share the same Roll_no, so Roll_no is a key attribute for the Student entity set. In an ER diagram, a key attribute is shown by underlining its name inside the oval. This is the ER-diagram view of the primary key we met in Part 2.

With entities, relationships, and now attributes in hand, you can describe exactly what data an ER diagram holds. Next in the series we look at how entities relate in numbers — the cardinality of a relationship.