ABSTRACT
Entity-relationship diagrams were first proposed as a means of quickly obtaining, with minimum effort, a good sense of the structure of a database. An entity-relationship diagram is an excellent tool for planning and designing a database, particularly when used in conjunction with data normalization. Before creating an entity-relationship diagram, the analyst must have at least a preliminary sense of the system’s logical entities, attributes, and data structures. An entity is an object about which data are stored. Consequently, a primary objective of entity-relationship modeling is to convert one-to-one and many-to-many relationships into one-to-many relationships. One solution is to create a new entity that has a one-to-many relationship with both original entities. Many-to-many relationships must be resolved, so add a new entity called Item ordered to the model, yielding two one-to-many relationships. Generally, entities that share a one-to-one relationship are really the same entity and should be merged unless there is a good reason to keep them separate.
