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Normalization not only makes your use of the database more efficient, but it also reduces the likelihood of data corruption. If you kept the customer s name in both the Customers table and the Orders table, you would run the risk that a change in one table might not be reflected in the other. Thus, if you changed the customer s address in the Customers table, that change might not be reflected in every row in the Orders table (and a lot of work would be necessary to make sure that it was reflected). By keeping only the CustomerID in Orders, you are free to change the address in Customers, and the change is automatically reflected for each order. Just as C# programmers want the compiler to catch bugs at compile time rather than at runtime, database programmers want the database to help them avoid data corruption. The compiler helps avoid bugs in C# by enforcing the rules of the language (for example, you can t use a variable you haven t defined yet). SQL Server and other modern relational databases avoid bugs by enforcing constraints that you define. For example, the Customers database marks the CustomerID as a primary key. This creates a primary key constraint in the database, which ensures that each CustomerID is unique. If you were to enter a customer named Liberty Associates, Inc., with the CustomerID of LIBE, and then tried to add Liberty Mutual Funds with a CustomerID of LIBE, the database would reject the second record because of the primary key constraint.
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Relational databases use declarative referential integrity (DRI) to establish constraints on the relationships among the various tables. For example, you might declare a constraint on the Orders table that dictates that no order can have a CustomerID unless that CustomerID represents a valid record in Customers. This helps avoid two types of mistakes. First, you can t enter a record with an invalid CustomerID. Second, you can t delete a customer record if that CustomerID is used in any order. The integrity of your data and its relationships is thus protected.
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The most popular language for querying and manipulating databases is Structured Query Language (SQL), usually pronounced sequel. SQL is a declarative language, as opposed to a procedural language, and it can take awhile to get used to working with a declarative language when you are used to languages such as C#. The heart of SQL is the query. A query is a statement that returns a set of records from the database. The queries in Transact-SQL (the version used by SQL Server) are very similar to the queries used in LINQ (as you ll see in the next chapter), though the actual syntax is slightly different. For example, you might like to see all the CompanyNames and CustomerIDs of every record in the Customers table in which the customer s address is in London. To do so, you d write this query:
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Select CustomerID, CompanyName from Customers where city = 'London'
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This returns the following six records as output:
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CustomerID ---------AROUT BSBEV CONSH EASTC NORTS SEVES CompanyName ---------------------------------------Around the Horn B's Beverages Consolidated Holdings Eastern Connection North/South Seven Seas Imports
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You can also sort the results based on a field:
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Select CustomerID, CompanyName from Customers where city = 'London' order by CompanyName
SQL is capable of much more powerful queries. For example, suppose the Northwind manager would like to know what products were purchased in July 1996 by the customer Vins et alcools Chevalier. This turns out to be somewhat complicated. The Order Details table knows the ProductID for all the products in any given order. The Orders table knows which CustomerIDs are associated with an order. The Customers table knows the CustomerID for a customer, and the Products table knows the product name for the ProductID. How do you tie all this together Here s the query:
select o.OrderID, productName from [Order Details] od join orders o on o.OrderID = od.OrderID join products p on p.ProductID = od.ProductID join customers c on o.CustomerID = c.CustomerID where c.CompanyName = 'Vins et alcools Chevalier' and orderDate >= '7/1/1996' and orderDate < '8/1/1996'
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