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This result does not include E13 because of the null value in the commission column.To ensure that the row with the null value is included, we need to look at the individual fields.From our COMPANY database example, The primary key is a candidate key that is selected by the database designer to be used as an identifying mechanism for the whole entity set.
Important points to note include: is a collection of entities of an entity type at a particular point of time.
For example, in the COMPANY database, a Spouse entity is existence -dependent on the Employee entity.
You should also be familiar with different kinds of entities including independent entities, dependent entities and characteristic entities. There are a few types of attributes you need to be familiar with.
An entity’s existence is dependent on the existence of the related entity.
It is existence-dependent if it has a mandatory foreign key (i.e., a foreign key attribute that cannot be null).
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Company database examples include: A one to many (1: M) relationship should be the norm in any relational database design and is found in all relational database environments. Figure 8.7 shows the relationship of one of these employees to the department.