Working with Transactions with an SQL Server Database

Transactions provide a way for you to group together database executions as a group so that they succeed or fail together. For example, if you had an e-commerce site you may have code that allows visitors to add a quantity of an item to their shopping cart. When you do that you also want to remove the number of items ordered from your inventory.


Therefore, you have two actions that you need to execute. You want to add items to a shopping cart and you want to remove items from inventory.

These executions need to happen as a group. You don't want to add items to the shopping cart if something goes wrong with removing them from inventory. And the opposite is also true.

Therefore, the database executions need to be grouped in a Transaction. This technique shows you

how to use a Transaction object with an SQL Server database.

This ASP.NET page contains SQL Delete statements that delete records from the Employees table. But the records are not deleted because they are in a transaction and the transaction is not committed to the database.

The code that performs this task fires when the ASP.NET page loads:

Sub Page_Load(ByVal Sender as Object, ByVal E as EventArgs) Within that procedure you will need a Connection object and a Command object:

Dim DBConn as SQLConnection Dim DBDelete As New SQLCommand

You will also need a Transaction object:

Dim DBTrans As SQLTransaction

You start by connecting to the SQL Server database:

DBConn = New SQLConnection("server=localhost;"
& "Initial Catalog=TT;" _
& "User Id=sa;" _
& "Password=yourpassword;")
and opening that connection:
DBConn.Open()

You then start a transaction by calling the BeginTransaction method of the Connection object. That method returns an open transaction, which is placed into the local Transaction object:

DBTrans = DBConn.BeginTransaction()

The Command object will connect to the database through the Connection object:

DBDelete.Connection = DBConn

It will also use the Transaction object:

DBDelete.Transaction = DBTrans

Next, two records are deleted and executed from the Employees table:

DBDelete.CommandText = "Delete From Employees
& "Where EmpID = 1" DBDelete.ExecuteNonQuery() DBDelete.CommandText = "Delete From Employees
& "Where EmpID = 2"DBDelete.ExecuteNonQuery()

But the records are not actually deleted from the database since the RollBack method of the Transaction object is called:

DBTrans.RollBack()
lblMessage.Text = "No action was taken."

The RollBack method causes the queries that were executed within the Transaction object to be cancelled.

You could instead call the Commit method:

'DBTrans.Commit()

['his method causes the pending execute statements to be executed as a group so that they fail or succeed together.

Deleting SQL Server Data, ASP.net

If you have code on your ASP.NET pages that allows visitors to enter a record, or where you add records through code without their input, you will likely need to delete records from your pages. This technique shows you how to use a SQL Delete query to delete records from an SQL Server table.

This technique displays a list of employees from an SQL Server table called Employees. Visitors select a name and then click the button. When they do that the employee selected is deleted from the database.

Defined on the page is this DropDownList control that will display the employee names:

dropdownlist id="ddlEmps" datatextfield="EmpName" datavaluefield="EmpID" runat="server"

and this Button control is defined:

button
id="butDelete" text="Delete" onclick="SubmitBtn_Click"
runat="server"
/ >

When the page first loads, the following code will populate the DropDownList control:

Sub Page_Load(ByVal Sender as Object, ByVal E as EventArgs)
If Not IsPostBack Then
Dim DBConn as SQLConnection
Dim DBCommand As SQLDataAdapter Dim DSPageData as New DataSet
DBConn = New SQLConnection("server=localhost;" _
& "Initial Catalog=TT;" _
& "User Id=sa;" _
"Password=yourpassword;") DBCommand = New SQLDataAdapter _ ("Select LastName + ' + FirstName "
& "as EmpName, EmpID " _
& "From Employees " _
& "Order By LastName, FirstName", DBConn)
DBCommand.Fill(DSPageData, _ "Employees")
ddlEmps.DataSource = _
DSPageData.Tables("Employees").DefaultView
ddlEmps.DataBind()
End If
End Sub

and this procedure fires when the Button control is clicked:

Sub SubmitBtn_Click(Sender As Object, E As EventArgs)

When that happens, you need a Connection and a Command object:

Dim DBConn as SQLConnection Dim DBDelete As New SQLCommand

You start by connecting to the SQL Server database:

DBConn = New SQLConnection("server=localhost;"
& "Initial Catalog=TT;" _
& "User Id=sa;"
"Password=yourpassword;")

Then the SQL Delete statement that will delete the record selected in the DropDownList control is placed into the CommandText property of the Command object:

DBDelete.CommandText = "Delete From Employees " _
& "Where EmpID = " & ddlEmps.SelectedItem.Value

The connection to the database for the Command object is made though the Connection object:

DBDelete.Connection = DBConn DBDelete.Connection.Open

and the offending record is deleted:

DBDelete.ExecuteNonQuery()