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What Are Primary Keys and Foreign Keys
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You can see in the previous tables that some columns have the letters (PK), which stands for primary key. In Figure 8-1, primary key columns are marked with a yellow key. Some other columns are identified with (FK) to indicate that these columns are foreign keys. Let s start by talking about the primary key.
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A primary key is a value that is used to uniquely identify a specific row in a table. A primary key has the following attributes: Can be composed of one or more column names. When it s composed of more than one column, it s called a composite key. Is often a numeric field. Is often generated by the RDBMS, in which case it s called a surrogate key. A surrogate key is frequently (but not always) a sequential number. A surrogate key is also called an identity in SQL Server 2005. An identity starts at a set number, called the identity seed, and increments by another set number, called the identity increment. For example, if you
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create a table named Product, you can have a column named ProductID that is set as an identity, and you can set the identity seed to 1 with an identity increment to 1. When the first row is created in the Product table, the ProductID will be generated by the RDBMS and set to 1. The following row will have a ProductID that is set to 2, and so forth. Should be as small as possible but large enough to support the number of rows it will represent. Is immutable, meaning its value should never change. Is also a natural key when the key has a logical relationship with the rest of the columns in the table. For example, if you had a book table, the ISBN number could be used as a primary key because it uniquely identifies one book. It would be an advantage compared with a generated key because it would take less space and has to exist anyway! Is also used to relate two tables together. In our Product table example, ProductID is the primary key. At design time, it will also be an identity. You can claim that the product number could be a primary key and you could be right but in certain scenarios a product number could be used twice. For example, suppose you have product #FG-001 with a revision 1.0. In time, you change the product because of customer complaints and give it a revision 2.0. You want your customers to continue to order the same product number for many business reasons. In your database, you would retire the product revision 1.0 by perhaps changing a column named Active, and you would then add another row in your table with the new product details, including revision 2.0, and set it to Active. Why can t you use the same row Let s assume that six months after creating the new product revision, you want to create a graph to determine whether your changes to the product mean you had fewer returns from your customers. It would be difficult to come up with good data if you had only one row for the product, but it would be fairly easy to do if you have two rows because they would be unique in the database, with each having a different ProductID. In the OrderDetail table, you have a composite primary key that is a combination of OrderID and OrderDetailID. This means these two columns would ensure the uniqueness of a row in the OrderDetail table. In the OrderHeader table, OrderID is the primary key.
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