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CHAPTER 3 DATABASE DESIGN AND CREATION
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it would need to be supplied by a number of different depots, which would typically be in different locations.
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Figure 3-3. Many-to-many relationship When building relationships within a database, it is necessary to have a foreign key. I covered foreign keys briefly earlier in the chapter; let s take a closer look at them in the next section.
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A foreign key is any key on a child table where a column, or a set of columns, can be directly matched with exactly the same number and information from the master table. By using this foreign key, you can build up the data to return via a relationship. However, a foreign key does not have to map to a primary key on a master table. Although it is common to see a foreign key mapped to a primary key, as long as the key in the master table that is being mapped to is a unique key, you can build a relationship between a master table and a child table. The whole essence of a foreign key lies in its mapping process and the fact that it is on the child table. A foreign key will exist only when a relationship has been created from the child table to the parent table. But what exactly are the master table and the child tables To demonstrate, let s refer back to our relationship examples. Take, for example, the one-to-many relationship. The master table would be on the left-hand side, or the one side of the relationship, and the child table would be on the right-hand side, or the many side of the relationship (see Figure 3-4). There is one final point to mention concerning foreign keys, relationships, and the master and child tables. It is totally possible for the master table and the child table to be the same table, and for the foreign key and the unique key to both be defined within the same table. This is called a self-join or a reflexive relationship. You don t tend to see this much within a database, as it is quite an unusual situation, although you could use it to ensure that the data in one column exactly matches the information in another column, just as in any other join. For example, say you have a table built around customers, and you have two columns, one of which is a parent customer ID, which holds an ID for the head office and is used to link all the branches. If the head office is also seen as valid branch of the conglomerate, the second column could be the specific branch ID, and you could put a link between these two columns so that there is still a valid link for the head office as a branch as well (see Figure 3-5). Another example is in an employees table where all employees reside, with a self-join from an employee back to his or her manager.
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CHAPTER 3 DATABASE DESIGN AND CREATION
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Figure 3-4. Foreign key
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Figure 3-5. Foreign keys in same table Now that we ve looked at relationships, let s move on to cover how to normalize the database.
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Normalization
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Normalizing a database is the science of reducing any duplication of data within tables. You can then build multiple tables related to one another through keys or indexes. The removal of as much duplication of data will lead to smaller, more compact databases. There will be a reduced chance of confusion over which column holding the same data is correct or should
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CHAPTER 3 DATABASE DESIGN AND CREATION
be modified, and there will also be less overhead involved in having to keep multiple columns of data up to date.
Note Just a reminder that we re still in the logical phase of building our solution, and we re not ready to
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