Question

我正在创建一个帐户系统，在这里我使用SELECT * FROM查询来读取数据库中的所有数据，以查看输入的用户名是否已经存在但是这只返回最后一个用户数据库，因此如果输入的用户与排除上次保存的帐户的任何其他用户匹配，则不会识别用户已经存在，因此创建具有相同用户名的帐户。下面是我的子例程代码。任何人都可以帮我找到我做错了什么

Dim conn As New OleDbConnection Dim myqry As String = Nothing Dim mycmd As New OleDbCommand Dim mydr As OleDbDataReader

Private Sub btn_createAccount_Click(sender As System.Object, e As System.EventArgs) Handles btn_createAccount.Click

    'Connecting to the database
    Try
        With conn
            If .State = ConnectionState.Open Then .Close()
            .ConnectionString = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=Database.accdb"
            .Open()

        End With

    Catch ex As Exception
        MessageBox.Show("Unable to connect", "error", MessageBoxButtons.OK, MessageBoxIcon.Error)
    End Try


    Dim student_Name As String
    Dim student_Username As String
    Dim student_Password As String
    Dim student_Group As String

    student_Name = txt_firstname.Text & " " & txt_surname.Text
    student_Username = LCase(txt_Username.Text)
    student_Password = txt_password.Text
    student_Group = cbo_tutorGroup.SelectedItem


    'This chunk of code is reading the username column in the student account table in my database and doing a read to see if the inputted username is already existent in the table.
    myqry = "SELECT * FROM TblStudents"
    mycmd = New OleDbCommand(myqry, conn)
    mydr = mycmd.ExecuteReader
    While mydr.Read
        Dim user As String = mydr("studentUser").ToString
        If user = student_Username Then
            MsgBox("Username already exists, please choose another")


        Else

            'If the username is not taken, the account credentials will be stored for use when logging in
            Dim sqlQry As String
            sqlQry = "INSERT INTO TblStudents(studentName, tutorGroup, studentUser, studentPass) VALUES('" & student_Name & "','" & student_Group & "','" & student_Username & "','" & student_Password & "')"

            Dim cmd As New OleDbCommand(sqlQry, conn)
            cmd.ExecuteNonQuery()

            MsgBox("Your account has successfully been created")


            Login_Student.Show()
            Me.Close()
            Exit Sub
        End If
    End While

Answer 1

我认为你误解了你的结果。查询可能会返回许多行，但如果第一个与您的条件（user = student_Username）不匹配，则插入记录并退出该过程。

为什么不检查您的特定用户是否存在：

SELECT * FROM TblStudents WHERE studentUser = :student_Username

而不是检查每个学生？

从数据库（SELECT * FROM）中选择数据时，只读取最后一行

1 个答案: