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A database key is a way to identify a set of rows in a table. The primary key for a table is a way to uniquely identify a particular row in the table. Often the primary key is a single column, such as CustomerId or EmployeeId, which could uniquely identify a row in the Customers or Employees tables. However, it s desirable that the primary key for a table can span multiple columns. In other words, the combination of OrderNumber and ItemNumber uniquely iden tifies a particular item purchased within an order. A secondary key is different from the primary key in that it need not uniquely identify a single row. In fact, secondary keys are useful for locating groups of rows that have something in common, such as all the customers in a particular city or all the orders that were placed on the same date.
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Microsoft Office Excel 2003 Programming Inside Out A foreign key is merely a secondary key in one table that is also the primary key in another table. Foreign keys are useful for identifying a group of data that is related, such as the collec tion of orders that was placed by a particular customer or the item numbers that comprise a single order.
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Indexes Save Time Primary keys and foreign keys are often used to optimize database access. Database designers use the keys to create indexes on the tables, which can significantly speed up access to a particular set of rows. Whenever possible, you should use indexes as part of the search criteria when retrieving data from a database.
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Some databases, including Microsoft Access, allow you to define an identity column. An iden tity column (called an AutoNumber column in Access) automatically generates a new value when a row is inserted into the table. This type of column is extremely useful as the primary key for a table because the database guarantees that the value is always unique.
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Databases are typically organized around the client/server concept. (See Figure 22-2.) This model assumes that the database server is separate from the database program that accesses the server. A database program uses a particular application programming interface (API), which in turn, communicates with the database server.
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Client Server
Figure 22-2. server. Databases are typically organized as a database client talking to a database
Excel relies on an API called Active Data Objects (ADO) to access databases. ADO is a stan dard component in Microsoft Windows, so it s always available to your program. Using ADO, you can connect to a variety of different databases, including Access, SQL Server, Oracle, and others. Note Although Access doesn t rely on the traditional client/server model, you still use the ADO interface to connect to an Access database. Once you ve established a connection with a database server with ADO, you can execute SQL statements to return data to your application or to make changes to the data stored in a table.
Excel and the Structured Query Language
Manipulating Databases with SQL
SQL provides a rich collection of statements for manipulating data in a relational database. In practice, most of these statements apply to creating or modifying tables or other database objects stored in the database, which leaves only four main statements that the average pro grammer really needs to understand in order to write database programs.
The Select statement specifies the rows of data you want to retrieve from the database. The Delete statement specifies the rows of data you want to remove from the database. The Insert statement specifies the rows of data that you want to add to the database. The Update statement specifies how to change existing rows of data in the database.
For instance, here s a Select statement that retrieves some rows of customer information:
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