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The way you work with data tables in a relational database is a bit different from the way you usually work on paper. A fundamental requirement in relational databases is that each data row in a table must be uniquely identifiable. This makes sense because you usually save records into a database so that you can retrieve them later; however, you can t always do that if each table row doesn t have something that makes it unique. For example, suppose you add another record to the department table shown previously in Figure 3-6, making it look like the table shown in Figure 3-7.
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Figure 3-7. Two departments with the same name Look at this table, and then find the description of the Costume Hats department. Yep, we have a problem two departments with the same name Costume Hats (the name isn t unique). If you queried the table using the name column, you would get two results. To solve this problem, you use a primary key, which allows you to uniquely identify a specific row out of many rows. Technically, the primary key is not a column itself. Instead, the PRIMARY KEY is a constraint that when applied on a column guarantees that the column will have unique values across the table.
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CHAPTER 3 CREATING THE PRODUCT CATALOG: PART I
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Note Applying a PRIMARY KEY constraint on a field also generates a unique index created on it by default. Indexes are objects that improve performance of many database operations, dramatically speeding up your web application (you ll learn more about this later in the Indexes section of this chapter).
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Constraints are rules that apply to data tables and make up part of the data integrity rules of the database. The database takes care of its own integrity and makes sure these rules aren t broken. If, for example, you try to add two identical values for a column that has a PRIMARY KEY constraint, the database refuses the operation and generates an error. We ll do some experiments later in this chapter to show this.
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Note A primary key is not a column but a constraint that applies to that column; however, from now
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on and for convenience, when referring to the primary key, we ll be talking about the column that has the
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PRIMARY KEY constraint applied to it.
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Back to the example, setting the name column as the primary key of the department table would solve the problem because two departments would not be allowed to have the same name. If name is the primary key of the department table, searching for a product with a specific name will always produce exactly one result if the name exists, or no results if no records have the specified name.
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This is common sense, but it has to be said: a primary key column will never allow NULL values. Tip
An alternative solution, and usually the preferred one, is to have an additional column in the table, called an ID column, to act as its primary key. With an ID column, the department table would look like Figure 3-8.
Figure 3-8. Adding an ID column as the primary key of department The primary key column is named department_id. We ll use this naming convention for primary key columns in all data tables we ll create. There are two main reasons it s better to create a separate numerical primary key column than to use the name (or another existing column) as the primary key:
CHAPTER 3 CREATING THE PRODUCT CATALOG: PART I
Performance: The database engine handles sorting and searching operations much faster with numerical values than with strings. This becomes even more relevant in the context of working with multiple related tables that need to be frequently joined (you ll learn more about this in 4). Department name changes: If you need to rely on the ID value being stable in time, creating an artificial key solves the problem because it s unlikely you ll ever want to change the ID. In Figure 3-8, the primary key is composed of a single column, but this is not a requirement. If the primary key is set on more than one column, the group of primary key columns (taken as a unit) is guaranteed to be unique, but the individual columns that form the primary key can have repeating values in the table. In 4, you ll see an example of a multivalued primary key. For now, it s enough to know that they exist.
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