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Figure 1-4
Movie relation with Copy Number added
Sometimes a multivalued attribute is disguised as multiple attributes. For example, Figure 1-4 could be changed to have separate attributes (columns) for up to three languages per movie, called Language 1, Language 2, and Language 3. However, they would still be considered multivalued attributes but in a special form called a repeating group, which is also forbidden in rst normal form. A repeating group is logically no different than one multivalued attribute. In fact, repeating groups often present more maintenance problems than multivalued attributes because a column must be added whenever you want to add more values than the original designer anticipated (such as a fourth language for a movie). Relational databases expect all rows in a table to have the same number of columns, but you can have as many rows as you wish in a table. Therefore, the trick is to take repeating columns and repeating values within columns and turn them into repeating rows in another table, and this is exactly what the rst normal form process instructs you to do. To transform unnormalized relations into rst normal form, you must move multivalued attributes and repeating groups to new relations. Because a repeating group is a set of attributes that repeat together, all attributes in a repeating group should be moved to the same new relation. However, a multivalued attribute (individual attributes that have multiple values) should be moved into its own new relation rather than combined with other multivalued attributes in the new relation.
Relational Database Concepts
The procedure for moving a multivalued attribute or repeating group to a new relation is as follows: 1. Create a new relation with a meaningful name. Often it makes sense to include all or part of the original relation s name in the new relation s name. 2. Copy the unique identi er from the original relation to the new one. The data depended on this identi er in the original relation, so it must depend on the same key in the new relation. This copied identi er will become a foreign key in the new relation. 3. Move the repeating group or multivalued attribute to the new relation. (The word move is used because these attributes are removed from the original relation.) 4. Form a unique identi er in the new relation by adding attributes to the unique identi er that was copied from the original relation. As always, be certain that the newly formed unique identi er has only the minimum attributes needed to make it unique. If you move a multivalued attribute, which is basically a repeating group of only one attribute, it is that attribute that is added in forming the unique identi er. This will seem odd at rst, but the unique identi er copied from the original relation is not only a foreign key to the original relation, but also usually part of the unique identi er (primary key) in the new relation. This is quite normal. Also, it is perfectly acceptable to have a relation where all the attributes are part of the unique identi er (that is, there are no non-key attributes). 5. Optionally, you may choose to replace the primary key with a single surrogate key attribute. If you do so, you must keep the attributes that make up the natural primary key formed in steps 2 and 4. Figure 1-5 shows the result of converting the relation shown in Figure 1-4 to rst normal form. Note the following: I took a bit of shortcut with the unique identi er in the new Movie Language relation. The languages in which a movie is available apply to the movie in general, not to individual copies. Notice that the list of languages does not vary in the duplicate rows for the same movie in Figure 1-4. Therefore, the Copy Number part of the unique identi er in the Movie relation was not copied to the new Movie Language relation. Had I done so, it would end up presenting a second normal form problem in the new relation that I would only have to x in the next normalization step. You ll nd that experienced database designers often synthesize the three normal forms simultaneously and simply rewrite original relations in third normal form. With practice, you ll be able to do the same.
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