如果不使用vb​​.net插入，如何檢查記錄是否存在？

Question

我堅持檢查數據庫中是否存在記錄。這很容易在PHP中，但我找不到一個很好的教程如何在vb.net中做到這一點。如果數據庫中不存在，我想從文本框中插入值。

這裏是我的代碼：

Using SQLConnection As New MySqlConnection(connString) 


     Using sqlCommand As New MySqlCommand() 
      With sqlCommand 
      'check if record exist 

      'if not execute these 
       .CommandText = "INSERT INTO bookrecords (Title, Author, Edition, Publisher, ISBN) values (@title,@author,@edition,@publisher,@isbn)" 
       .Connection = SQLConnection 
       .CommandType = CommandType.Text 
       .Parameters.AddWithValue("@title", txtTitle.Text) 
       .Parameters.AddWithValue("@author", txtAuthor.Text) 
       .Parameters.AddWithValue("@edition", txtEdition.Text) 
       .Parameters.AddWithValue("@publisher", txtPublisher.Text) 
       .Parameters.AddWithValue("@isbn", txtISBN.Text) 

      End With 


      Try 
       SQLConnection.ConnectionString = "Server=localhost;Database=booksdb;Uid=root;Pwd=;" 
       SQLConnection.Open() 
       sqlCommand.ExecuteNonQuery() 
       iReturn = True 
       'MessageBox.Show("Connection Opened") 

      Catch ex As MySqlException 
       MessageBox.Show("Error: " & ex.ToString()) 
       iReturn = False 
      Finally 
       SQLConnection.Close() 
       'MessageBox.Show("Connection Closed") 
      End Try 

     End Using 
    End Using 

我只想@isbn是用於確定是否記錄已經存在的關鍵。

Answer 1

顯而易見的事情是（無需觸及數據庫模式），將計數查詢發送到數據庫以獲取所需的ISBN。
這可以通過的ExecuteScalar來完成相應的查詢

Using con = new MySqlConnection(connectionString) 
    Using sqlCommand As New MySqlCommand() 
     With sqlCommand 
      .Connection = con 
      .Parameters.AddWithValue("@isbn", txtISBN.Text) 
      .CommandText = "SELECT COUNT(*) FROM bookrecords WHERE ISBN = @isbn" 
     End With 
     con.Open() 
     Dim result = sqlCommand.ExecuteScalar() 
     if Convert.ToInt32(result) = 0 Then 
      ' ISBN doesn't exist .... 
      sqlCommand.Parameters.Clear() 
      ' rest of your insert code ... 

如果命令返回零計數記得清除參數集合隨後插入查詢

Answer 2

你可以在東西一個查詢做像這樣：

.CommandText = "INSERT INTO bookrecords (Title, Author, Edition, Publisher, ISBN) " & _ 
"SELECT * FROM (SELECT @title, @author, @edition, @publisher, @isbn AS ISBN) t " & _ 
"WHERE NOT EXISTS(SELECT ISBN FROM bookrecords b WHERE t.ISBN = b.ISBN) "