As you begin to design the music database, you are either creating a new system or you are transferring data from a non-relational system to ALLBASE/SQL. Regardless of where you're coming from, you need to take a comprehensive look at all the information needs that will be served by the database system. This means identifying the elements for which you need to store and retrieve information.
These elements are known as entities. A list of entities grows out of studying how the data is used by its owners.
Listing Entities |
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By approaching the problem intuitively, you can probably identify four different categories of information required by the radio station:
These are the entities in the data.
Listing Attributes |
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Next, you need to define the attributes of each entity, which are the useful pieces of information to be stored in tables. In addition to supplying informational detail, some attributes are used to distinguish one entity from another. As you subdivide your data, make sure that for each entity you define, at least one attribute can uniquely identify an instance of the entity. This attribute or group of attributes is known as a key.
Table 2-1 Attributes for Four Entities
| Album Entity | Selection Entity | Composer Entity | Station Log Entity |
|---|
| Album Name | Selection Title | Name | Selection Title |
| Medium | Composer Name | Date of Birth | Start Time |
| Album Cost | Timing | Birthplace | End Time |
| Recording Company | Performers | Comment | Announcer |
| Date Recorded | Comment | | Comment |
| Manufacturer's Code | | | |
| Comment | | | |
As the design evolves, entities eventually become database tables, and attributes eventually become columns. Note, however, that at this stage you have not yet identified the form of the database tables. Before you can do that, you need to identify relationships.
