Question

Imports System.Data.OleDb
Public Class Form1
    Dim con As New OleDbConnection
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        con.ConnectionString = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=c:\users\sjmi.alfie\documents\visual studio 2010\Projects\Sample Ko\WindowsApplication4\MySample.accdb"
        If con.State = ConnectionState.Closed Then
            con.Open()
            MsgBox("connection is open", vbInformation)
        Else
            MsgBox("connection is close", vbCritical)
        End If
    End Sub
    Private Sub closeButton_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles closeButton.Click
        Me.Close()
    End Sub

    Private Sub regButton_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles regButton.Click

        Dim strsql As String
        Dim numrows As Integer
        strsql = "SELECT COUNT(*) FROM Sample WHERE StudNo = " & TextBox1.Text & ""
        Dim objcmd As New OleDbCommand(strsql, con)
        numrows = objcmd.ExecuteScalar()  <<<< Error "Data type mismatch in criteria." 

        If numrows > 0 Then
            MsgBox("Record Exists", vbInformation, "Add")
        Else
            Dim myadapter1 As New OleDbDataAdapter("INSERT INTO [Sample] ([StudNo], [FirsName], [LastName]) VALUES ('" & TextBox1.Text & "', '" & TextBox2.Text & "', '" & TextBox3.Text & "')", con)
            Dim mytable1 As New DataTable
            myadapter1.Fill(mytable1)
            MsgBox("Success!!")
        End If
        con.Close()
    End Sub
End Class

请帮帮我。感谢

Answer 1

请改为尝试：

strsql = "SELECT COUNT(*) FROM Sample WHERE StudNo = ?"
Dim objcmd As New OleDbCommand(strsql, con)
objcmd.Parameters.AddWithValue("?", TextBox1.Text)
numrows = Convert.ToInt32(objcmd.ExecuteScalar())

它被称为“参数化查询”，它是做你想做的事情的首选方式。

检查数据库VB.Net中是否存在数据

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