訪問數據庫選擇用VB.NET代碼

Question

我有一個表2場「醫學生」和「Listino」，所以我只需要選擇Listino那裏的領域「醫學生」 = Medhi

MEDICO  LISTINO 
--------------------- 
Medhi   Gheller 
Ashi   Cadon 
Pamdo   Gheller 


result: 
Gheller 

因爲Medhi有 'Listino' Gheller

下面的代碼

Public DBPath As String = "Listini.accdb" 
    Public StringaConn As String = "Provider=Microsoft.ACE.OLEDB.12.0; Data Source= " & DBPath & "" 
    Public DBConn As New OleDbConnection(StringaConn) 

    Dim Medico As String = "Medhi" 
    DBConn.Open() 
    Dim DBCommAssociazione As OleDbCommand = DBConn.CreateCommand 
    DBCommAssociazione.CommandType = CommandType.Text 
    DBCommAssociazione.CommandText = "SELECT Listino FROM Associazioni WHERE Medico=Medhi" 

    Dim reader As OleDbDataReader 
    reader = DBCommAssociazione.ExecuteReader 

    While reader.Read() 
     Me.ListBox1.Items.Add(reader(0)) 
     Me.ListBox1.Items.Add(reader(1)) 

    End While 

    DBConn.Close() 

謝謝。

Answer 1

你缺少科茨''查詢WHERE Medico='Medhi'，但我建議你使用參數，以避免SQL注入，像這樣：

Dim Medico As String = "Medhi" 
DBConn.Open() 
Dim DBCommAssociazione As OleDbCommand = DBConn.CreateCommand 
DBCommAssociazione.CommandType = CommandType.Text 
DBCommAssociazione.CommandText = "SELECT Listino FROM Associazioni WHERE Medico=@Medico" 
DBCommAssociazione.Parameters.AddWithValue("@Medico", Medico) 

Dim reader As OleDbDataReader 
reader = DBCommAssociazione.ExecuteReader 

While reader.Read() 
    Me.ListBox1.Items.Add(reader(0)) 
End While 

Answer 2

醫學生是一個文本字段。要搜索此列上的值，應該在單引號之間放置常量值。但是在更現實的情況下，您需要根據用戶的輸入搜索特定的Medico。

所以，你應該使用如下代碼

Public DBPath As String = "Listini.accdb" 
Public StringaConn As String = "Provider=Microsoft.ACE.OLEDB.12.0; Data Source= " & DBPath & "" 

Using DBConn = New OleDbConnection(StringaConn) 
Using DBCommAssociazione = DBConn.CreateCommand 
    DBConn.Open() 
    Dim Medico = WhateverMethodYouHaveToReturnAMedicoFromUserInput() 
    DBCommAssociazione.CommandText = "SELECT Listino FROM Associazioni WHERE Medico=@medico" 

    DBCommAssociazione.Parameters.Add("@medico", OleDbType.VarWChar).Value = Medico 
    Using reader = DBCommAssociazione.ExecuteReader 
     While reader.Read() 
      Me.ListBox1.Items.Add(reader(0)) 
     End While 
    End Using 
End Using 

還要注意查詢只返回一個字段。在While循環中，您嘗試讀取兩個字段（reader（0）和reader（1）），這會給您一個ArgumentOutOfRangeException。 DataReader在一次處理一條記錄，並且只有在調用Read方法時才加載新記錄。