Does every entity need a primary key?

Does every entity need a primary key?

The primary key is an attribute or a set of attributes that uniquely identify a specific instance of an entity. Every entity in the data model must have a primary key whose values uniquely identify instances of the entity. The value must be unique for each instance of an entity.

What is partial completeness?

Partial completeness (symbolized by a circle over a single line) means that not every supertype occurrence is a member of a subtype; that is, there may be some supertype occurrences that are not members of any subtype. A single horizontal line under the circle represents a partial completeness constraint.

How are supertypes and subtypes represented in database?

A supertype is a generic entity type that has a relationship with one or more subtypes. A subtype is a sub-grouping of the entities in an entity type that is meaningful to the organization and that shares common attributes or relationships distinct from other subgroups.

What is class hierarchy in DBMS?

It breaks an entity into multiple entities from higher level (super class) to lower level (subclass). The class vehicle can be specialized into Car, Truck and Motorcycle ( Top Down Approach) Hence, vehicle is the superclass and Car, Truck, Motorcycle are subclasses.

What is disjoint rule?

The disjoint rule forces subclasses to have disjoint sets of entities. The overlap rule forces a subclass (also known as a supertype instance) to have overlapping sets of entities. Completeness constraints – decide whether a supertype instance must also be a member of at least one subtype.

What is the difference between disjoint and overlapping?

Disjoint events are events that don’t have any outcomes in common. Overlapping events are events that have outcomes in common. You can do this by drawing a Venn diagram that shows if any events are common to the two sets. The answer is that these events are overlapping as there are numbers common to both events.