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我卡在检查数据库中是否存在记录。这在 php 中很容易，但我在 vb.net 中找不到一个好的教程。如果数据库中不存在，我想从文本框中插入值。

这是我的代码：

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

我只是希望@isbn 成为确定记录是否已经存在的关键。

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2 回答 2

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显而易见的事情（不涉及数据库模式）是向数据库发送计数查询以获取所需的 ISBN。
这可以通过 ExecuteScalar 适当的查询来完成

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 ...

如果命令返回计数为零，请记住为后续插入查询清除参数集合

于 2013-03-10T10:04:06.737 回答
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您可以在一个查询中执行此操作，如下所示：

.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 ) "
于 2015-06-12T08:18:28.117 回答