Tài liệu Create a New Table with Data from Existing Tables doc

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:
• Using the EXIST statement with a SELECT statement, querying the system table
called sysobjects
• Using an IF statement to conditionally execute another command, in this case the
DROP TABLE statement
Tip
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.

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.