What s Under the Hood in Visual Basic .NET

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What s Under the Hood
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People often believe that because SQL Server Express is free it must not be as good as SQL Server. But actually SQL Server Express Edition is SQL Server. Before getting into details, let s first establish that SQL stands for Structured Query Language. The database understands this language and allows you to manipulate the data and structures with it. In this section, we outline the key difference between SQL Server Express and the other SQL Server versions. SQL Server is installed as a windows service, which is a piece of software running in the background. There are many services installed with an operating system that perform work in the background you may be unaware of. For example, the indexing service improves performance when searching for files on your computer, or you may have antivirus software installed on your computer that will also be running as a service in the background. In the same manner, SQL Server Express runs as a service in the background waiting for connections. What makes SQL Server unique when compared to other software is that it may be installed multiple times on the same computer. There are very good reasons for this. With the full version of SQL Server, you can specify what central processing unit (CPU) the service should use. If you install multiple instances of SQL Server on a computer that has multiple CPUs, you can tell each instance to use a specific CPU and enhance performance. You will notice in that last statement we used the word instance. Each time you install SQL Server on the same computer, you install what is called an instance of SQL Server. In fact, you can install up to 50 instances on one computer. Each instance must be given its own name. When you install SQL Server Express Edition for the first time, you install the default instance. In previous versions of SQL Server, the default instance did not have a name; it was simply referred to as the default instance. However, in SQL Server Express, the default instance is called SQLEXPRESS. You refer to any instance of SQL Server by its fully qualified name, which is broken down as <computer name>\<instance name>. If you are dealing with a local machine, as you will be doing throughout this book, you can use a dot as the computer name. Now that we have discussed SQL Server Express instances and SQL Server connections, let s put the two together. As we explained earlier, SQL Express runs as a windows service. It is simply running in the background while we are typing this chapter, minding its own business,
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C H A P T E R 3 INTRODUCING SQL SERVER EXPRESS
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waiting for a connection. Applications use some type of programming interface to communicate with SQL Server. A programming interface is nothing more than a library that you can use to establish connections with SQL Server. One such library you will see a lot of in this book is ADO.NET 2.0. ADO.NET provides a few different ways to talk to SQL Server; however, in all cases, it requires information about where SQL Server is, what database you wish to use, and what identity it should use to talk to SQL Server. All these items are put together in what is called a connection string. You will see more on this later in the book when we introduce ADO.NET 2.0. Here is a sneak peek at what one connection string could look like: Server=.\SQLEXPRESS;Integrated Security=True;Database=MyDatabase.mdf In this connection string we are saying that we want to connect to a server called localhost\SQLEXPRESS, we want to use Windows Integrated Security, and we wish to use the MyDatabase.mdf database. You will see more about the security options and what the database name consists of in the next few sections. For now it is important just to understand that SQL Server is running in the background as a windows service waiting for a connection to be established. Once a connection is established, what happens next It would not serve a great purpose if applications were to simply connect to SQL Server and remain connected. More often than not, a connection is going to be established because someone is using an application to request data, update data, or perform any number of other operations available against the data. ADO.NET 2.0 provides many mechanisms to accomplish working successfully with a SQL Server database, but in the end, the commands sent to the database itself must be in the form of SQL, the language that SQL Server understands and can interpret. It uses this language to determine which actions to take against which data. All database engines on the market understand SQL. Although SQL is an industry-standardized language, most of the larger database engines, such as SQL Server and Oracle, have their own proprietary extensions to the SQL. Oracle has PL/SQL, and SQL Server uses Transact-SQL (T-SQL). T-SQL greatly enhances the power of what you can accomplish when working with a SQL Server database. A full discussion of T-SQL is a large subject and certainly outside the scope of this book; however, in a later section, we take a look at the basic SQL syntax that will be required throughout the book as you build your application. The inner workings of a database engine are extremely complex, and very large books have been written on the subject. This brief high-level explanation certainly does not do it justice, but for now, it is all you really need to know. At a high level, all versions of SQL Server work the same way. Of course, if they were all exactly the same, we wouldn t have so many different variations of it. Let s take a look now at the differences between SQL Server Express and the full Enterprise versions of SQL Server as well as the differences with Microsoft Database Engine (MSDE).
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