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Figure 1-1 MOVIE table listing
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Relational Database Concepts
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You have likely noticed the striking similarity between relational database tables and spreadsheets. However, as you will see in the remainder of this chapter, relational databases offer many more features and much greater exibility in organizing and displaying information.
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Relationships
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Relationships are the associations among relational database tables. While each relational table can stand alone, databases are all about storing related data. For example, you can store information about categories used by the video store to organize the inventory of movies in addition to the movies themselves. At the same time, you can store information about the copies of each video you have in the video store, including the date the copy was acquired and the format of the copy (DVD or VHS). By using relationships, you can tie the related tables together in a formal way that is easy to use when you want to combine data from multiple tables in the same database query but with the exibility to include only the information of interest. This ability to pick and choose the information you want from the database allows you to tailor the information in the database to the speci c needs of each individual or application that accesses the database. Figure 1-2 shows four tables from the video store database and the relationships among them in a format known as an Entity Relationship Diagram (ERD). ERDs provide an easy medium for showing the overall design of a relational database and are easily understood by both technical and nontechnical database users. Each rectangle in the diagram represents a relational table, with the name of the table
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MPAA_RATING <pk> MPAA RATING CODE MPAA_RATING_DESCRIPTION
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MOVIE <pk> MOVIE ID MOVIE_GENRE_CODE <fk1> MPAA_RATING_CODE <fk2> MOVIE_TITLE RETAIL_PRICE_VHS RETAIL_PRICE_DVD YEAR_PRODUCED
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MOVIE_GENRE <pk> MOVIE GENRE CODE MOVIE_GENRE_DESCRIPTION
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MOVIE_COPY <pk,fk> MOVIE ID COPY NUMBER <pk> DATE_ACQUIRED DATE_SOLD MEDIA_FORMAT
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Figure 1-2
Video store database ERD, partial view
SQL Demysti ed
appearing above the horizontal line and the columns in the table listed vertically in the main part of the rectangle. You may wish to compare the MOVIE table as shown in Figure 1-2 with the listing of the same table shown in Figure 1-1 to help you visualize the contents of the table. Each relationship is shown on the ERD as a line connecting two tables. Each end of a relationship line shows maximum cardinality of the relationship, which is the maximum number of rows in one table that can be associated with a given row in the table at the opposite end of the relationship line. The maximum cardinality may be one (where the line has no special symbol on its end) or many (where the line has a symbol called a crow s foot on the end, which looks like the line end splitting into three lines). Just short of the end of the line is another symbol that shows the minimum cardinality, which is the minimum number of rows of one table that can be associated with the table on the opposite side of the line. The minimum cardinality may be zero, denoted with a circle drawn on the line, or one, denoted with a short vertical line or tick mark drawn across the relationship line. For example, the relationship between the MPAA_RATING and MOVIE tables in Figure 1-2 is a one-to-many relationship, which means that each row in the MPAA_RATING table (the table on the one side, which is also called the parent table) can be associated with many rows in the MOVIE table (the table on the many side, which is also called the child table), but each row in the MOVIE table can be associated with only one row in the MPAA_RATING table. This should make sense because each movie released in the U.S. has only one rating, and each rating can be assigned to many different movies. I recognize that sometimes movies are recut to achieve a different rating, but this is easily handled by treating different versions as different movies, much as we do when a movie is remade using a different cast and crew. It is essential to consider such things because relational databases only support oneto-many relationships. The minimum cardinality indicates whether participation in a relationship is mandatory or optional. All of the relationships in Figure 1-2 are mandatory on the one side and optional on the many side, which is the most common form of relationship. Looking back at the relationship between the MPAA_RATING and MOVIE tables, this means that each row in the MOVIE table must have a matching row in the MPAA_RATING table at all times, but that a given row in the MPAA_ RATING table does not necessarily have to have a matching row in the MOVIE table at all times. If you wanted to allow movies to be in the video store inventory that did not have an MPAA rating assigned, the tick mark near the MPAA_RATING table end of the relationship line would show as a circle. While optional relationships on the one side of a relationship are relatively common, it is most unusual to have a mandatory relationship on the many side, which essentially means that the parent table must have at least one child in the database at all times. Consider the
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