The relationship between the two entities is one-to-one, but optional on both sides. In order to convert this relationship into a relational format, three relations will be needed, one for each entity and one for the relationship. In this case, two relations will be needed, one for each entity. The relationship could be mandatory for the first entity and optional for the second, or the other way around. There are therefore two possibilities for performers and agents. This part of an entity-relationship model can be converted into a single relation, Performer-details.
In this example, all performers must be represented by agents, and each performer has only one agent. The agents themselves need not be responsible for making bookings for performers, and can be involved in other activities. In the second example, an agent must represent a performer, but a performer does not need to have an agent. Here, the relationship is optional for a performer, but mandatory for an agent. These occur when a model suggests the existence of a relationship between entity types, but the pathway does not exist between certain entity occurrences. In this example, it is still necessary for performers to be represented by a number of agents, but the agents now have more flexibility as they do not have to make bookings for performers.
It is not directly supported by RDBMSs although it is directly supported by object DBMSs. To implement this strategy in an RDBMS a set of columns must be defined for the maximum possible number of related instances. The data model for such a domain would have an entity for Product related one to many to an entity for Ending Inventory. The Ending Inventory entity would need two attributes, Month and Ending Quantity.
Sometimes, engineers will branch out ER diagrams with additional hierarchies to add necessary information levels for database design. For example, they may add groupings by extend up with superclasses and down with subclasses. This means that it is possible to have a performer without an agent, and it is also permissible julie chrisley food blog for an agent to have no performers. All performer details will be stored in the Performers relation, and all agent data will be held in the Agent relation. Where there is a performer with an agent, this will be shown in the relation Works-with, which will represent the relationship between the two entities.
This chapter introduces the ideas of top-down database design, and provides the starting point in learning how to develop a database application. The chapter links closely with the others covering database design . Alternate keysare all candidate keys not chosen as the primary key. A candidate key is a simple or composite key that is unique and minimal.
