Building the Data Warehouse

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The other type of snapshot trigger is time, which is a predictable trigger, such as the end of the day, the end of the week, or the end of the month.


The snapshot triggered by an event has four basic components:


■■ A key


■■ A unit of time


■■ Primary data that relates only to the key


■■ Secondary data captured as part of the snapshot process that has no direct relationship to the primary data or key


Of these components, only secondary data is optional.


The key can be unique or nonunique and it can be a single element of data. In a typical data warehouse, however, the key is a composite made up of many elements of data that serve to identify the primary data. The key identifies the record and the primary data.


The unit of time, such as year, month, day, hour, and quarter, usually (but not always) refers to the moment when the event being described by the snapshot has occurred. Occasionally, the unit of time refers to the moment when the capture of data takes place. (In some cases a distinction is made between when an


secondary data



key



nonkey


primary


data


Figure 3.34 A data warehouse record of data is a snapshot taken at one moment in time and includes a variety of types of data.

event occurs and when the information about the event is captured. In other cases no distinction is made.) In the case of events triggered by the passage of time, the time element may be implied rather than directly attached to the snapshot.

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