Getting a Database Schema

This article has been excerpted from the book "A Programmer's Guide to ADO.NET in C#".

 

Usually, when you work with databases, you already known the database schema such as database tables, table columns, add the column properties. What if you don't know the database schema, and you need to know database tables, their columns, and column properties programmatically?

 

In this sample, I'll show you how to access a database schema programmatically. As you can see from figure 10-36, I created a Windows application with one text box, three buttons, and two list boxes. The Browser button lets you browse .mdb database on your machine. The GetTables button then reads the database tables and adds them to the first list box. The Get Table Schema button returns table columns and the properties of the selected table on the list box.

 

 

Figure 10-36: Getting a database schema programmatically

 

Listing 10-11 shows the source code for this application. As you can see, the BrowseBtn_Click handler browsers Access database on the machine fills the selected database name to the text box, and sets dbName as the database name, which is a string type of variable defined as follows:

// Define variables
private string dbName = "";

Listing 10-11: Reading a database schemas programmatically

private void BrowseBtn_Click(object sender, System.EventArgs e) {
    OpenFileDialog fdlg = new OpenFileDialog();
    fdlg.Title = "C# Corner Open File Dialog";
    fdlg.InitialDirectory = @ "c:\";
    fdlg.Filter = "All files (*.*)|*.mdb|" + "MS-Access Database files (*.mdb)|*.mdb";
    fdlg.FilterIndex = 2;
    fdlg.RestoreDirectory = true;
    if (fdlg.ShowDialog() == DialogResult.OK) {
        textBox1.Text = fdlg.FileName;
        dbName = fdlg.FileName;
    }
}

The GetOleDbSchemaTable method of OleDbConnection returns a data table object containing database tables. As you can see from the GetTableBtn_Click handler in Listing 10-12, I set the dataset to the left list box with the DisplayMember property as TABLE_NAME. I set DesplayMember because I want to show only one column of the data table in the list box. 

 

Listing 10-12: Getting database tables from a SQL Server database 

private void GetTablesBtn_Click(object sender, System.EventArgs e) {
   // Connection string
   string strDSN = "Provider=Microsoft.Jet.OLEDB.4.0; " + "Data Source=" + dbName;
   try {
       // Create a connection and open it
       OleDbConnection conn = new OleDbConnection(strDSN);
       conn.Open();
       // Call GetOleDbSchemaTable to get the schema data table
       DataTable dt = conn.GetOleDbSchemaTable(OleDbSchemaGuid.Tables, new object[] { null, null, null, "TABLE" });
       // Set DataSource and Displaymember properties
       // of the list box control
       listBox1.DataSource = dt.DefaultView;
       listBox1.DisplayMember = "TABLE_NAME";
       // Close the connection
       conn.Close();
   } catch (Exception exp) {
       MessageBox.Show(exp.Message.ToString());
   }
}

GetSchemaBn_Click listed in Listing 10-13 is the event handler that returns the columns and their properties of a database table. You read the database table using SELECT * and use DataTable to get columns. The DataColumn class defines a unique, column data type, column name, and so on.

 

Listing 10-13: Getting a database table schema 

private void GetSchemaBtn_Click(object sender, System.EventArgs e) {
    // Get the selected item text of list box
    string selTable = listBox1.GetItemText(listBox1.SelectedItem);
    // Connection string
    string strDSN = "Provider=Microsoft.Jet.OLEDB.4.0;" +
        "Data Source=" + dbName;
    try {
        // Create and open connection
        OleDbConnection conn = new OleDbConnection(strDSN);
        conn.Open();
        string strSQL = "SELECT * FROM " + selTable;
        //Create data adapter
        OleDbDataAdapter myCmd = new OleDbDataAdapter(strSQL, conn);
        // Create and fill dataset
        DataSet dtSet = new DataSet();
        myCmd.Fill(dtSet);
        DataTable dt = dtSet.Tables[0];
        // Add items to the list box control
        listBox2.Items.Add("Column Name, DataType, Unique," +
            "AutoIncrement, AllowNull");
        listBox2.Items.Add("=============");
        foreach(DataColumn dc in dt.Columns) {
            listBox2.Items.Add(dc.ColumnName + " , " + dc.DataType +
                " , " + dc.Unique + " ," + dc.AutoIncrement + " ," + dc.AllowDBNull);
        }
        // close connection
        conn.Close();
    } catch (Exception exp) {
        MessageBox.Show(exp.Message.ToString());
    }
}

Selecting Distinct Records

 

It isn't hard to select unique rows from a database table using SELECT DISTINCT, but sometimes people get stuck on this. So, it's not a bad idea to talk about the procedure.

 

You can use the SELECT DISTINCT SQL statement to select distinct records from a database. This is useful when you want to return only one record corresponding to a criterion. Listing 10-14 returns distinct records from the Employees table ordered by the LastName.

 

Listing 10-14: Selecting distinct rows from a database table

private void button1_Click(object sender, System.EventArgs e) {
    // create a connection object
    string ConnectionString = @ "Provider=Microsoft.Jet.OLEDB.4.0;" +
        "Data Source=c:\\Northwind.mdb";
    OleDbConnection conn = new OleDbConnection(ConnectionString);
    // Open the connection
    if (conn.State != ConnectionState.Open)
        conn.Open();
    string sql = "SELECT DISTINCT(LastName)" +
        "FROM Employees ORDER BY LastName";
    OleDbDataAdapter da = new OleDbDataAdapter(sql, conn);
    DataSet ds = new DataSet();
    da.Fill(ds, "Employees");
    dataGrid1.DataSource = ds.DefaultViewManager;
    // Close the connection
    if (conn.State == ConnectionState.Open)
        conn.Close();
}

Conclusion

 

Hope this article would have helped you in understanding getting a Database Schema. See other articles on the website also for further reference.

 

This essential guide to Microsoft's ADO.NET overviews C# then leads you toward a deeper understanding of ADO.NET.