Building the Data Warehouse

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The Midlevel Data Model


After the high-level data model is created, the next level is established—the midlevel model, or the DIS. For each major subject area, or entity, identified in the high-level data model, a midlevel model is created, as seen in Figure 3.13. The high-level data model has identified four entities, or major subject areas. Each area is subsequently developed into its own midlevel model.


Interestingly, only very rarely are all of the midlevel models developed at once. The midlevel data model for one major subject area is expanded, then a portion of the model is fleshed out while other parts remain static, and so forth.


Shown in Figure 3.14, four basic constructs are found at the midlevel model:



DIS


the relationship between the ERD and the DIS Figure 3.13 Each entity in the ERD is further defined by its own DIS,





primary grouping of data


secondary grouping of data


■■ A primary grouping of data ■■ A secondary grouping of data


■■ A connector, signifying the relationships of data between major subject areas


■ “Type of” data


The primary grouping exists once, and only once, for each major subject area. It holds attributes that exist only once for each major subject area. As with all groupings of data, the primary grouping contains attributes and keys for each major subject area.


The secondary grouping holds data attributes that can exist multiple times for each major subject area. This grouping is indicated by a line emanating downward from the primary grouping of data. There may be as many secondary groupings as there are distinct groups of data that can occur multiple times.

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