Dependency, Existence, Identifiers

Define the dependency , the existence ,identifier and referential dependencies

Dependencies are checks or constraints actually.

1. Existence Dependency: This is the type of dependency which exists when one entity instance needs instance of another entity for its existence. For example, an entity MOVIE COPY can not exist without entity MOVIE. Here in this example, entity type MOVIE is considered as parent entity and MOVIE COPY as child entity.
2. Identifier Dependency: It means that the dependent entity (MOVIE COPY) in case of existence dependency does not have its own identifier and any external identifier is used to pick data for that entity. And to define a key in this entity the key of the parent entity (MOVIE) is to be used in the key for this entity may be used as composite keys.
3. Referential Dependency: This is the situation when the dependent entity (MOVIE COPY) has its own key for unique identification but the key used to show the reference with the parent entity (MOVIE) is shown with the help of an attribute of the parent entity. Means to show the link of the parent entity with this entity there will be an attribute and a record in this entity will not exist without having a record in the parent entity. Despite of having its own identifier attribute. This type of identifier or attribute in the weak entity (MOVIE COPY)  is known as foreign key.