What is the difference between entities and attributes

These are described below. Independent entities , also referred to as kernels, are the backbone of the database. They are what other tables are based on.

Kernels have the following characteristics:. Dependent entities , also referred to as derived entities , depend on other tables for their meaning. These entities have the following characteristics:. Characteristic entities provide more information about another table. Each entity is described by a set of attributes e. Each attribute has a name, and is associated with an entity and a domain of legal values. However, the information about attribute domain is not presented on the ERD.

In the entity relationship diagram, shown in Figure 8. There are a few types of attributes you need to be familiar with. Some of these are to be left as is, but some need to be adjusted to facilitate representation in the relational model. This first section will discuss the types of attributes. Later on we will discuss fixing the attributes to fit correctly into the relational model. Simple attributes are those drawn from the atomic value domains; they are also called single-valued attributes.

Composite attributes are those that consist of a hierarchy of attributes. Using our database example, and shown in Figure 8. Multivalued attributes are attributes that have a set of values for each entity.

Derived attributes are attributes that contain values calculated from other attributes. An example of this can be seen in Figure 8. Age can be derived from the attribute Birthdate.

In this situation, Birthdate is called a stored attribute, which is physically saved to the database. An important constraint on an entity is the key. The key is an attribute or a group of attributes whose values can be used to uniquely identify an individual entity in an entity set.

A candidate key is a simple or composite key that is unique and minimal. It is unique because no two rows in a table may have the same value at any time. It is minimal because every column is necessary in order to attain uniqueness. The primary key is a candidate key that is selected by the database designer to be used as an identifying mechanism for the whole entity set.

Attributes that identify a person would include such things as social security number or any combination of letters and numbers that uniquely identify a person. Attributes that describe entities are called non-key attributes. Attributes that identify entities entity identifiers are called key attributes. The next lesson explains the purpose of entity identifiers. Before moving on to the next lesson, click the Exercise link below to test your understanding of entities and attributes.

All these entities have some attributes or properties that give them their identity. An entity set is a collection of similar types of entities. An entity set may contain entities with attribute sharing similar values.

For example, a Students set may contain all the students of a school; likewise a Teachers set may contain all the teachers of a school from all faculties.

Entity sets need not be disjoint. Entities are represented by means of their properties, called attributes. All attributes have values. What is Entity — Definition, Functionality 2. What is Attribute — Definition, Functionality 3. For example, assume a university management system.

It stores information about students, lecturers, courses, exams, etc. The Student, lecturer, course, and exam become the tables in the database. A record in student table is called an entity. It represents a single student object. Similarly, a record in lecturer table is an entity. It represents a one lecturer object. These entities are called strong entities. There is another entity type as a weak entity.