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the entire primary key. Figure 1-6 shows the solution. All the attributes that depend only on Movie ID are now in a relation (named Movie) with Movie ID as the unique identi er. Those that depend on the combination of Movie ID and Copy Number are in a relation (named Movie Copy) with Movie ID and Copy Number as the unique identi ers. The Movie Language relation was already in second normal form because it has no non-key attributes and thus remains unchanged.
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To understand third normal form, you must rst understand transitive dependency. An attribute that depends on an attribute that is not the unique identi er (primary key) of the relation is said to be transitively dependent. Looking at the Movie relation in Figure 1-6, notice that Genre Description depends on Genre Code, and MPAA Rating Description depends on MPAA Rating Code. The danger of leaving these descriptions in the Movie relation is that you end up making Genre and MPAA rating arti cially dependent on Movie, which leads to all three of the data anomalies introduced earlier in this chapter. A relation is said to be in third normal form if it meets both the following criteria: The relation is in second normal form. There is no transitive dependence (that is, all the non-key attributes depend only on the unique identi er). To transform a second normal form relation into third normal form, simply move any transitively dependent attributes to relations where they depend only on the primary key. Be careful to leave the attribute on which they depend in the original relation as the foreign key. You will need it to reconstruct the original user view via a join. Incidentally, any attributes that are easily calculated are removed as third normal form violations. For example, if on a sales transaction, Quantity Purchased times Price Each yields Total Paid, it s easy to see that Total Paid is dependent on Quantity Purchased and Price Each. Assuming all three of those would be dependent on the unique identi er of the relation that contains them, it s easy to see that Total Paid (the calculated result) is, in fact, transitively dependent on the other two attributes. Figure 1-7 contains the solution in third normal form. Note that you have created new relations for MPAA Rating and Movie Genre, moved the descriptions to the new relations, and left the code attributes (MPAA Rating Code and Movie Genre Code) in the Movie relation as foreign keys. Many database designers call relations like MPAA Rating and Movie Genre lookup tables or code tables because their main usage is to look up descriptions for the codes that are stored in the primary key
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Movie: MPAA MPAA MOVIE GENRE GENRE RATING RATING ID CODE DESCRIPTION CODE DESCRIPTION Under 17 requires accompanying parent or adult guardian Under 17 requires accompanying parent or adult guardian Parents strongly cautioned Parents strongly cautioned MOVIE TITLE YEAR PRODUCED
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1 Drama
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Action and Adventure
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MOVIE LANGUAGE CODE ID en 1 1 fr en 2 2 fr es 2 en 3 en 4 fr 4 Movie Copy: MOVIE COPY DATE MEDIA RETAIL DATE ID NUMBER ACQUIRED SOLD FORMAT PRICE 1 1 19.96 1/1/2005 DVD 1 2 1/10/2005 19.96 DVD 2 2 1/10/2005 VHS 15.95 1 3 1/10/2005 1/30/2005 DVD 29.99 3 2 DVD 2/15/2005 29.99 1 4 DVD 2/15/2005 19.99
Figure 1-6
Second normal form solution
Movie: MOVIE ID MOVIE GENRE CODE 1 Drama 2 ActAd 3 Comdy 4 ActAd Movie Copy: COPY DATE MOVIE NUMBER ACQUIRED ID 1 2 2 3 3 4 MPAA Rating: MPAA RATING CODE PG-13 R Movie Genre: MOVIE GENRE CODE ActAd Comdy Drama MOVIE GENRE DESCRIPTION Action and Adventure Comedy Drama MPAA RATING DESCRIPTION Parents strongly cautioned Under 17 requires accompanying parent or adult guardian 1 1 2 1 2 1 1/1/2005 1/10/2005 1/10/2005 1/10/2005 2/15/2005 2/15/2005 DATE SOLD MEDIA FORMAT DVD DVD VHS 1/30/2005 DVD DVD DVD MPAA RATING CODE R R PG-13 PG-13 MOVIE TITLE Mystic River The Last Samurai Something s Gotta Give The Italian Job RETAIL PRICE VHS 58.97 15.95 14.95 11.95
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