mysql - 如果用户输入数据库中不存在的值，它将使用 VB.Net 通过 MySQL 设置文本标签

Question

具体的查询是什么？我能想到的最好办法是，如果用户输入的内容为空，则设置 TextLabel 消息，但如果用户输入的内容错误，则不设置 TextLabel 消息。我不知道正确的查询。我尝试了 NULL 但我不知道它的确切查询，所以我尝试了 = Nothing 但它什么也没做。这是我的代码:

Private Sub Button2_Click(sender As System.Object, e As System.EventArgs) Handles Button2.Click
    Dim connString As String = "server=localhost;userid=root;password=;database=cph;Convert Zero Datetime=True"
    Dim sqlQuery As String = "SELECT emp_firstnm, emp_midnm, emp_lastnm FROM employee_table WHERE emp_no = @empno"

    If txtEmpno.Text = Nothing Then
        Label4.Text = "No such employee exists"
    Else

        Using sqlConn As New MySqlConnection(connString)
            Using sqlComm As New MySqlCommand()
                With sqlComm
                    .Connection = sqlConn
                    .CommandText = sqlQuery
                    .CommandType = CommandType.Text
                    .Parameters.AddWithValue("@empno", txtEmpno.Text)
                End With
                Try
                    sqlConn.Open()
                    Dim sqlReader As MySqlDataReader = sqlComm.ExecuteReader()
                    While sqlReader.Read()
                        Label4.Text = sqlReader("emp_firstnm").ToString() & " " & sqlReader("emp_midnm").ToString() & " " & sqlReader("emp_lastnm").ToString()
                    End While
                Catch ex As MySqlException
                    MessageBox.Show(ex.Message)
                Finally
                    sConnection.Dispose()
                End Try
            End Using
        End Using
    End If


End Sub

Answer 1

使用String.IsNullOrEmpty .

If (String.IsNullOrEmpty(txtEmpno.Text)) Then

或者

If (txtEmpno.Text = String.Empty) Then

...相当于

If (txtEmpno.Text = "") Then

String 是一个引用类型(类)，并且可以设置为Nothing。但是，根据设计，标签将始终返回空字符串 ""，而不是 null。

txtEmpno.Text = Nothing
MessageBox.Show(String.Format("{0}, {1}", (txtEmpno.Text Is Nothing), (txtEmpno.Text = "")))

将输出:

False, True