Chapter 12: Advanced Data Access

locally, and you need to be aware of all the possibilities in order to be able to make an informed decision about which mode to use.

Working with Data Sets and Data Tables

We’ve been working with databases for a while now. You first learned about the SqlDataReader, and used it to populate your controls. Then, in the first half of this chapter, we gained first-hand experience with the data source controls, which can automate many features for you. Let’s learn about one more technique you can use to get data to your visitors.

I know that all these options can be confusing at first—you need to play around with all of them before you can become an experienced ASP.NET developer!

The DataSet object is at the center of Microsoft’s model for presenting disconnected data. Disconnected data (data that resides in memory and is completely independent of the data source) gives us a raft of new opportunities of developing desktop and web apps.

In Chapter 9, you learned about the role that data readers play in relation to applications and database data. Whenever we need to access data from a database, we create a connection object and a command object. These two objects are used together to create a data reader object, which we can use within our application by binding it to a data control for presentation purposes. Figure 12.18 illustrates this process.

So, what’s the problem? Well, while being the fastest way to retrieve data from a database, data readers can’t be used to carry out any significant work—you can’t use them to sort, filter, or page through the data. As the arrows in Figure 12.18 show, data readers present a forward-only stream of data to the application: you can’t go back to a previous record, or reuse that data reader somewhere else. Moreover, once a data reader has been created, it remains tied to the database. This means that you can’t make multiple requests to a database using the same data reader.

Data sets, on the other hand, are much more flexible. Imagine a virtual database that you’re free to use in code whenever and however you wish. That’s a data set. As we’ll see in the next section, data sets have all the bells and whistles that databases offer, including tables, columns, rows, relationships, and even queries! A data set is a memory-resident copy of database data, so, once a data set has been created, it can be stored in memory and its ties to the database can be broken. Then, when you need to work with the data, you don’t need to re-query

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Working with Data Sets and Data Tables

Figure 12.18. Retrieving data using a data reader

Figure 12.19. Breaking the data set’s ties to the data source once it has been created

the database—you simply retrieve the data from the data set again and again.

Figure 12.19 illustrates this point.

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Chapter 12: Advanced Data Access

Figure 12.20. Multiple pages making multiple requests from the same data set

An even greater advantage is that data sets can be shared among multiple requests, as illustrated in Figure 12.20.

What this means is that you simply need to create the data set once per request. Once the data set has been created, it can be used by many different pages, all of which may make multiple—and even different—requests to that data set.

However, data sets require much more memory and resources than do data readers. A data reader simply keeps track of its position within the results of a query, whereas a data set can contain a local copy of an entire database. The larger the amount of data kept in the data set, the more memory the data set uses.

When deciding whether to use data readers or data sets, you need to consider the purpose for which you need the data. This decision is important, because it affects:

resources consumed on the database server

resources consumed on the application server

the overall application architecture

If you’re only reading the data, using a data reader can make sense. If you also need to update, insert, or delete data, or you need to process the data within your code, data sets might be of more help.

This section will teach you everything you need to know to begin working with data sets.

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What is a Data Set Made From?

What is a Data Set Made From?

A data set comprises many parts, all of which are required for its usage. The following classes will be introduced and discussed in this section:

DataSet

SqlDataAdapter

DataTable

DataColumn

DataRow

DataRelation

DataView

We need to use most of these classes in order to work with data sets. For instance, the SqlDataAdapter class acts as the communication point between the DataSet and the database. The data adapter knows how to fill a DataSet with data from the data source; it also knows how to submit to the data source any changes you’ve made to a DataSet.

Data Adapters

Of all these objects, only SqlDataAdapter depends on a particular data provider, and as such, it is interchangeable with other classes that work with data providers, including OracleDataAdapter, OleDbDataAdapter, and so on. As this class is the bridge between the database and your local data storage objects (the other classes mentioned), it makes sense for the class to be database-specific. The DataSet, DataTable, and other classes, store the data locally and don’t need to be aware of the source of that data.

A DataSet will always contain at least one DataTable, but it can contain many. These DataTables contain DataColumns and DataRows. If we needed to establish a relationship between multiple DataTables within a DataSet, we’d use DataRelations. Finally, we’d create DataViews to query the DataSet.

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