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Logical Data Entities
To logically order the above information, each attribute must be analyzed
in terms of how it will be used. These logical groupings of attributes
are called entities.
The program director needs to know what albums are available, the
selections on each, how long each selection is, and some general
information about the style of performance. The announcers need to know
which selections are on which albums, how long each selection is, and
some information about the composers and artists. The station log will
track all music played, which announcer played it, and when the selection
started and finished.
The attributes can be divided into the following four categories of
information as required by the radio station:
* Album information
* Selection information
* Composer information
* Station log information
These constitute our four data entities.
Next, you need to identify which attributes belong to which entity.
Below is a table that shows one way of subdividing the attributes amongst
the entities. For each entity, one attribute in the list will uniquely
identify an instance of the entity. This attribute is known as a key.
Attributes for Four Entities
-------------------------------------------------------------------------------------------------
| | | | |
| Album | Selection | Composer | Station Log |
| Entity | Entity | Entity | Entity |
| | | | |
-------------------------------------------------------------------------------------------------
| | | | |
| Album Name | Selection Title | Name | Selection Title |
| | | | |
| Medium | Composer Name | Date of Birth | Start Time |
| | | | |
| Album Cost | Duration | Birthplace | End Time |
| | | | |
| Recording Company | Performers | Comment | Announcer |
| | | | |
| Date Recorded | Comment | | Comment |
| | | | |
| Manufacturer's Code | | | |
| | | | |
| Comment | | | |
| | | | |
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As the design evolves, entities become tables and attributes become
columns. Recall that tables and columns are SQL concepts. Using
TurboIMAGE/XL terminology, the entities become data sets and the
attributes become data items. The key, in TurboIMAGE/XL, is also known
as a search item.
Examining the data, you will see some overlapping attributes. These will
help you define the relationships between the entities.