6.7 Create a New Table with Data from Existing Tables

I often need to create new tables from existing data. If the table already exists, I need to delete the old table first. How do I do this using T-SQL?

Technique

To perform these tasks, you will create two T-SQL statements and use them in one Command object. Here are the two statements that will be used:

IF EXISTS (SELECT * FROM sysobjects
        WHERE id = object_id(N'[Northwind].[dbo].[MyProdAndCat]'))
        DROP TABLE [Northwind].[dbo].[MyProdAndCat]

SELECT Categories.CategoryName, Products.ProductName INTO MyProdAndCat
       FROM Categories INNER JOIN ProductsON Categories.CategoryID =
       Products.CategoryID"

The first statement checks for the existence of the particular table you will be creating, in this case MyProdAndCat. This statement demonstrates a couple of techniques that you can use in T-SQL:

Tip

graphics/tip_icon.gif

Now that you have learned this technique, you will want to use it repeatedly. Make sure you mark this page!


The last statement uses an inner join to join two tables displaying the CategoryName and ProductName. The clause that creates the new table is this:

INTO MyProdAndCat

This tells SQL Server to create a new table called MyProdAndCat from the SELECT statement that is specified.

Steps

Open and run the Visual Basic .NET-Chapter 6 solution. From the main form, click on the button with the caption How-To 6.7. You will see the SQL string specified in the "Technique" section displayed in a label. If you click the Execute button, the new table is generated a SELECT statement is executed, and the results are displayed in the DataGrid object (see Figure 6.8).

  1. Create a Windows Form. Then place the controls listed in Table 6.7 with the following properties set, as displayed in Figure 6.8.

    Table 6.7. Control Property Settings for This How-To

    Object

    Property

    Setting

    Label

    Text

    SQL Statement

    Label

    Name

    lblSQLString

    Label

    Text

    Results

    Button

    Name

    btnExecute
     

    Text

    Execute

    DataGrid

    Name

    dgResults

  2. Add the code in Listing 6.16 to the Load event of the form. (Double-click on the form to bring up the code.)

    Listing 6.16 frmHowTo6_7.vb: Storing the SQL Statement in the lblSQLString Label to Display and Use Later
    Private Sub frmHowTo6_7_Load(ByVal sender As System.Object, _
                ByVal e As System.EventArgs) Handles MyBase.Load

    '-- Build the SQL String that returns cities that
'    have more than one customer in them.
    Dim strSQL As String
    strSQL = "IF EXISTS (SELECT * from sysobjects " & vbCrLf
    strSQL &= "  WHERE id = object_id(N'[Northwind].[dbo].[MyProdAndCat]'))" & _
vbCrLf
    strSQL &= "  DROP Table [Northwind].[dbo].[MyProdAndCat]" & _
vbCrLf & vbCrLf
    strSQL &= "SELECT Categories.CategoryName, Products.ProductName" & vbCrLf
    strSQL &= "INTO MyProdAndCat" & vbCrLf
    strSQL &= "FROM Categories INNER JOIN Products" & vbCrLf
    strSQL &= "ON Categories.CategoryID = Products.CategoryID"
    Me.lblSQLString.Text = strSQL

End Sub

  3. Add the following code in Listing 6.17 to the Click event of btnExecute. This code creates Connection and Command objects by using the T-SQL routine discussed in the "Technique" section. Then the code executes the query. Next, a select query is run against the new table, and the DataSource property is set to the data table that was filled.

    Listing 6.17 frmHowTo6_7.vb: Loading the Form
    Private Sub btnExecute_Click(ByVal sender As System.Object,
                ByVal e As System.EventArgs) Handles btnExecute.Click
        Dim dtResults As New DataTable()
        Try
            Dim ocnn As New OleDb.OleDbConnection(BuildCnnStr("(local)", _
                  "Northwind"))
            Dim ocmd As New OleDb.OleDbCommand(Me.lblSQLString.Text)
            ocmd.Connection = ocnn
            ocnn.Open()
            ocmd.ExecuteNonQuery()
            ocnn.Close()

            '-- Use the SQL String to build the data adapter
    '    and fill the data table.
            Dim odaResults As _
           New OleDb.OleDbDataAdapter("Select * From MyProdAndCat",
               BuildCnnStr("(local)", "Northwind"))

            odaResults.Fill(dtResults)

        Catch excp As Exception
            MessageBox.Show(excp.Message)
            Exit Sub
        End Try

        '-- Assign the data table to the data grid's DataSource property
        Me.dgResults.DataSource = dtResults

End Sub
Figure 6.8. These results are based on a new table created by the SQL string that is displayed.

graphics/06fig08.jpg

Comments

You will probably want to go ahead and drop the new table after you are finished using it if you don't need to keep it around for any other purposes.