Interview Questions - DBMS

An entity is something that can be identified in the users' work environment, something that the users want to track. Entities of a given type are grouped into entity classes. An entity instance is the representation of a particular entity.

Entities have attributes. Attributes are properties that describe the entity's characteristics. Entity instances have identifiers. Identifiers are attributes that name, or identify, entity instances.

1:1 - a single entity instance of one type is related to a single-entity instance of another type.

1:N - a single entity instance of one type is related to many-entity instances of another type.

M:N - many-entity instances of one type relate to many-entity instances of another type.

The archetype/instance pattern occurs when one entity tracks occurrences of another entity. A common example is the relationship between CLASS and SECTION, where the actual occurrence of a class happens when students register for a SECTION of that CLASS. The archetype/instance pattern is implemented as an ID-dependent relationship.