Question

由于我早期问题here的帮助，我在构建的应用程序上取得了一些可靠的进展。我正在研究“创建用户”功能。我似乎无法弄清楚的是如何检查用户名是否已经在数据库中。请查看代码并让我知道我做错了什么。显然，这只是我需要帮助的部分。

ElseIf UserNameTextBox.Text = "SELECT * FROM Users WHERE Username = '" Then 

     MsgBox("Username Already Taken!") 
     Dim NEWUSER As New NewUser 
     NEWUSER.Show() 
     Me.Hide() 

Answer 1

由于这是相当一个“怎么办，我-进行-问题”，考虑他在你的文本框的用户名以下

• 的用户类型，并点击“创建用户”按钮
• 该程序从文本框中获取值，并在数据库上运行查询，以检查用户是否已经在分区中，如果情况是，在屏幕上显示错误或者建议新用户名，基于用户输入的用户名，您验证的用户名作为一种新的可能正确的值的情况下，
• 没有，你创建的用户，并让他知道，他的帐户创建得到了

你这样做之前，我们需要您的基本知识：

• 我如何查询数据库，并使用返回值
• 如何添加用户名到该查询，使之SQL注入安全
• 编程逻辑的基本知识，还可以帮助（虽然，你似乎有做到了这一点远...）

所以最终，它归结为：

Dim strUsername As String = usernametextbox.text 
Dim boolUsernameExists as Boolean = False 

If String.IsNullOrEmpty(strUsername) Then 
    ' do not allow empty usernames 
    Return 
End If 

Using dbConnection as OleDbConnection = new OleDbConnection("your-connection-string") 
    dbConnection.Open() 
    Using dbCommand as OleDbCommand = new OleDbCommand("select count(username) from users where username like ?", dbConnection) 
     dbCommand.Parameters.AddWithValue("@p1", strUsername) 
     Dim result As Integer = DirectCast(dbCommand.ExecuteScalar(), Integer) 
     If result > 0 Then 
      boolUsernameExists = true 
     End If 
    End Using 
    dbConnection.Close() 
End Using 

If boolUsernameExists Then 
    ' throw error, or do some other action 
    ... 
    ' and return 
    Return 
End If 
' If you end up here, your user doesn't exist, and it safe to create the new username 

我想指出的是，这是没有的编辑写的，可能有一些错误，不过，正如你指出的那样，纯粹是如何对进行-问题

Answer 2

cn.Open（）

Dim dr As OleDbDataReader 
    Dim cmd1 As New OleDbCommand("Select username from login where username= '" & TextBox2.Text & "'", cn) 
    dr = cmd1.ExecuteReader 
    If dr.Read = True Then 
     MsgBox("Please insert another username") 
    Else 
     If TextBox3.Text = TextBox4.Text Then 
      ' cn.Open() 
      Dim cmd As New OleDbCommand("insert into login values('" & TextBox2.Text & "','" & TextBox3.Text & "','" & TextBox4.Text & "')", cn) 
      cmd.ExecuteNonQuery() 
      MsgBox("sign up Successfully") 
      ' cn.Close() 
      TextBox2.Clear() 
      TextBox3.Clear() 
      TextBox4.Clear() 
      Form1.Show() 
      Me.Hide() 
     Else 
      MsgBox("Please insert same password & Confirm Password") 
     End If 

    End If 

    cn.Close() 

Answer 3

Imports System.Data.SqlClient 
Imports System.Data 


Public Class Forgetpassword 
    Dim cn As New SqlConnection("Data Source=DESKTOP-UIT47KQ;Initial Catalog=vishakha;Integrated Security=True") 
    Dim result As String 


    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click 
     cn.Open() 
     Dim cmd As New SqlCommand("Select answer,passw from login where squestion = '" & Label4.Text & "'", cn) 
     Dim dr As SqlDataReader 
     dr = cmd.ExecuteReader 
     If dr.Read = True Then 
      If TextBox1.Text = dr.Item("answer").ToString Then 
       Label1.Text = dr.Item("passw").ToString 
      Else 
       MsgBox("Please Enter Right Answer") 
      End If 
     Else 

     End If 
     cn.Close() 
    End Sub 

    Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click 
     cn.Open() 
     Dim cmd As New SqlCommand("select squestion from login where uid1='" & txtusername.Text & "'", cn) 
     Dim dr As SqlDataReader 
     dr = cmd.ExecuteReader 
     If dr.Read = True Then 
      Label4.Text = dr.Item("squestion").ToString 

     Else 
      MsgBox("Please Enter Right Username") 
     End If`enter code here` 
     cn.Close() 
    End Sub 



    Private Sub Button3_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button3.Click 
     Form1.Show() 
     Me.Hide() 

    End Sub 

    Private Sub Button4_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button4.Click 


    End Sub 
End Class