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So far, the focus in this chapter has been on the relation and how to normalize data. However, an important component of any relational database is how those relations are associated with each other. These associations, or relationships, link relations together in meaningful ways, which helps to ensure the integrity of the data so that an action taken in one relation does not negatively impact data in another relation. There are three primary types of relationships:
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One-to-one A relationship between two relations in which a tuple in the first relation is related to at most one tuple in the second relation, and a tuple in the second relation is related to at most one tuple in the first relation. One-to-many A relationship between two relations in which a tuple in the first relation is related to zero, one, or more tuples in the second relation, but a tuple in the second relation is related to at most one tuple in the first relation. Many-to-many A relationship between two relations in which a tuple in the first relation is related to zero, one, or more tuples in the second relation, and a tuple in the second relation is related to zero, one, or more tuples in the first relation.
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The best way to illustrate these relationships is to look at a data model of several relations (shown in Figure 1-6). The relations are named to make referencing them easier. As you can see, all three types of relationships are represented:
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A one-to-one relationship exists between the ARTIST_AGENCIES relation and the ARTIST_NAMES relation. For each artist listed in the ARTIST_AGENCIES relation, there can be only one matching tuple in the ARTIST_NAMES relation, and vice versa. This implies a business rule that an artist may work with only one agency at a time.
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A one-to-many relationship exists between the ARTIST_NAMES relation and the ARTIST_CDS relation. For each artist in the ARTIST_NAMES relation, zero, one, or more tuples for that artist can be listed in the ARTIST_CDS relation. In other words, each artist could have made zero, one, or more CDs. However, for each artist listed in the ARTIST_CDS relation, there can be only one related tuple for that artist in the ARTIST_ NAMES relation because each artist can have only tuple in the ARTIST_NAMES relation. A one-to-many relationship exists between the ARTIST_CDS relation and the COMPACT_ DISCS relation. For each CD, there can be one or more artists; however, each tuple in ARTIST_CDS can match only one tuple in COMPACT_DISCS because each CD can appear only once in the COMPACT_DISCS relation. A many-to-many relationship exists between the ARTIST_NAMES relation and the COMPACT_DISCS relation. For every artist, there can be zero, one, or more CDs, and for every CD, there can be one or more artists.
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Relational databases only support one-to-many relationships directly. A many-to-many relationship is physically implemented by adding a third relation between the first and second relation to create two one-to-many relationships. In Figure 1-6, the ARTIST_CDS relation was added between the ARTIST_NAMES relation and the COMPACT_DISCS relation. A one-to-one relationship is physically implemented just like a one-to-many relationship, except that a constraint is added to prevent duplicate matching rows on the many side of the relationship. In Figure 1-6, a unique constraint would be added on the ARTIST_ID attribute to prevent an artist from appearing with more than one agency.
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ARTIST_AGENCIES
ARTIST_ID AGENCY 10001 10002 10003 10004 10005 10006
One-to-many
ARTIST_CDS
One-to-many
COMPACT_DISCS
ARTIST_NAMES
ARTIST_ID ARTIST_NAME
ARTIST_ID CD_ID 10001 10002 10002 10003 10004 10005 99301 99302 99303 99304 99305 99306
CD_ID
CD_NAME
2305 2306 2306 2345 2367 2049
10002 10003 10004 10005 10006
Jennifer Warnes Joni Mitchell
99301 Famous Blue Raincoat 99302 Blue 99303 Court and Spark 99304 Past Light 99305 Kojiki 99306 That Christmas Feeling 99307 Patsy Cline: 12 Greatest Hits
William Ackerman
Kitaro Bing Crosby Patsy Cline
99307 10006 10006 99307
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