70 Part I EXAM PREPARATION or other unique
70 Part I EXAM PREPARATION or other unique data elements. Relationships define a real-world connection or link between one entity and another. In the modeling process, you attempt to discover how things and which things are related to one another. Within the business problem you are modeling, each object requires full definition, including how it is related to other objects. These relationships are usually defined as a numeric link connecting the entities together based on the number of data elements in one entity that are related to one or more full elements in another entity. It can be described as how many of one thing can relate to how many of something else. This property of a relationship is known as the cardinality of a relationship. In the Lloyd s Hospital example, patients buy or take drugs, so a Patient entity would probably be related to a Medicine entity. Relationships can be made from one entity to another in essentially three different ways. These different relations are one-to-one, one-tomany, and many-to-many. The relationships between two entities is often called a binary relationship. The following sections describe how to identify, apply, and choose the correct relationships between the different entities in your model. Although a binary relationship between two entities is common, there may be relationships between three entities (ternary), and more. Of course, an entity might be related to itself (unary or a self-referencing entity). One-To-One This type of relationship occurs when one row or data element of an entity is associated with only one row or element in the second entity. It is not surprising that one-to-one relationships are uncommon in the real world. They are used mostly when an entity has an extraordinarily large number of attributes, so the entity is split in two to make it more easy to manage, and applications perform better. An extra entity might be desired during the development of the physical storage locations of the data. By separating off seldom-used data from more frequently used information, faster data retrieval and updates can be accommodated. For example, employees may have pictures but the pictures are not often retrieved by a front-end appli
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