Entities Attributes and Relationships

Entities Attributes and Relationships are key elements in ER diagrams.

Entity

• An entity is a real world object of independent of another.
• In an ER diagram, entities are typically represented as rectangles as shown above.
• Examples – Student, Teacher, Employee, Product, Order

Mainly there are 2 types

1. Strong Entity
   • A strong entity is an entity that has a primary key attribute
   • It is not dependent on any other entity.
   • It can be uniquely identified by its own attributes.
2. Weak Entity
   • A weak entity is an entity that doesn’t have a primary key attribute of its own.
   • Its existence depends on another entity, called the owner entity.
   • Weak entities are typically identified by a partial key, and their primary key includes the primary key of the owner entity.

Best describes of Attributes

• In ER diagrams, attributes are properties or characteristics that describe the entities in the database.
• There are different types of attributes, each has a specific purpose when defining the database structure.
• Here are common types of attributes in an ER diagram.

1. Simple Attribute
   • A simple attribute cannot be divided any further, that means it is atomic. It represents a single value associated with an entity.
   • Example, Name, Age, studentID, address
2. Composite Attribute:
   • A composite attribute is an attribute that can be divided into smaller sub parts.
   • Example, name attribute can be composed of sub parts like first name, middle name and last name.
3. Derived Attribute
   • A derived attribute is an attribute whose value can be derived or calculated from another attribute (from stored attribute).
   • It is not stored physically because it can be computed when needed.
   • Example, “Age” might be a derived attribute based on the “Date of Birth.”
4. Multi valued Attribute
   • A multi-valued attribute can keep multiple values for a single entity.
   • For instance, an entity “Employee” can have a multi valued attribute called “Phone Number” to store multiple phone numbers associated with that Employee.
5. Key Attribute
   • A key attribute is an attribute that can uniquely identify an entity.
   • In an ER diagram, key attributes are often underlined.
6. Composite Key
   • A composite key is a key that consists of two or more attributes for the primary key.
   • It is used when a single attribute is not sufficient to uniquely identify an entity.
   • Composite keys are represented by underlining the combination of attributes in an ER diagram.

Relationship

• In ER diagrams, a relationship is a connection between two or more entities.
• It describes how data is related and interconnected in a database.
• Relationships are often categorized into different types based on the cardinality (example., 1:1, 1:M and M:N).

Weak Relationship

• A weak relationship is a type of relationship where the existence of one entity (the child entity) depends on another entity (the parent entity).
• This child entity has a primary key that is partially or totally derived from the parent entity. Without the parent entity, the child entity may not be uniquely identified.
• To represent a weak relationship, you can use a double diamond shape in the connecting line between the parent and child entities. Additionally, the child entity may have a double-lined border to indicate its weakness.