Beginning C# 3.0

14.4. Database Queries

Okay. . . so now you know how to create a database and its associated tables, add fields to a table, and add or update a record in a table. How do you retrieve the information in a useful way? Obviously, that's the "query" part of SQL. Figure 14-8 shows the form for generating a query.

Figure 14-8. Figure 14-8

The figure shows the state of the form after the Do Query menu option is selected. The form cannot be loaded if the user has not opened a database. The code that ensures the database is open is in the mnuQuery_Click() event code in frmMain:

private void mnuQuery_Click(object sender, EventArgs e)
{
    if (dbName == null)     // See if they opened a DB
    {
        MessageBox.Show("You must open a database first");
        return;
    }
    frmReport myFriendReport = new frmReport(dbName, tableName);
    myFriendReport.ShowDialog();
}

If variable dbName is not set, a message is issued asking the user to select a database. Assuming that he has selected a database, the form shown in Figure 14-8 appears. The listbox object on the left side of the figure lists the tables that are in the currently opened database. When the user double-clicks one of the table names, the code performs a nonrestrictive query on the database based upon the table selected. The SELECT statement is copied into the textbox and the query is run. A sample result is shown in Figure 14-9.

Figure 14-9. Figure 14-9

One ...