按当月获取记录的问题答案

【问题标题】：problem with getting record by current month按当月获取记录的问题
【发布时间】：2022-01-25 19:36:52
【问题描述】：

我试图在这里获取特定记录，但它总是返回空白

     Try
        cmd = con.CreateCommand()
        cmd.CommandType = CommandType.Text
        cmd.CommandText = "SELECT SUM(a.qty_sold) FROM sales AS a INNER JOIN transaction_history AS c ON c.Id = a.transaction_id WHERE MONTH(c.transaction.date) = MONTH(CURRENT_DATE()) AND YEAR(c.transaction.date) = YEAR(CURRENT_DATE())"
        cmd.ExecuteNonQuery()

        Dim dt As New DataTable()
        Dim da As New SqlDataAdapter(cmd)

        da.Fill(dt)

        Dim dr As SqlClient.SqlDataReader
        dr = cmd.ExecuteReader(CommandBehavior.CloseConnection)

        While dr.Read
            current_month_products_sold = dr.GetInt32(3).ToString()

        End While


    Catch ex As Exception

    End Try
    MessageBox.Show(current_month_products_sold)

    total_product_sold.Text = current_month_products_sold

我不知道是查询问题还是我提取数据的方式

【问题讨论】：

标签： vb.net visual-studio

【解决方案1】：

不要写空的Catch 块。他们只会吞下错误。

最好将数据库代码和用户界面代码分开。我创建了一个Function 来执行此操作。

命令和连接需要调用其Dispose 方法，因为它们使用在Dispose 方法中释放的非托管资源。 Using blocks 为我们处理关闭和处理，即使有错误。

在使用它们的方法中声明连接，以便可以处理它们。您可以将连接字符串传递给连接的构造函数。同样，将CommandText 字符串和连接传递给命令的构造函数。

由于我们只检索单个值，我们可以使用ExecuteScalar。这将返回结果集第一行的第一列，始终为Object。因此，CInt。我假设 qty_sold 字段是一个整数字段。

在尝试代码之前先在 SSMS 中测试您的 Select 语句。（SQL Server 管理系统）

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    Dim current_month_products_sold = ""
    Try
        current_month_products_sold = GetSalesSum().ToString
    Catch ex As Exception
        MessageBox.Show(ex.Message)
        Exit Sub
    End Try
    MessageBox.Show(current_month_products_sold)
    total_product_sold.Text = current_month_products_sold
End Sub

Private Function GetSalesSum() As Integer
    Dim RetVal As Integer
    Dim sql = "SELECT SUM(a.qty_sold) FROM sales AS a INNER JOIN transaction_history AS c ON c.Id = a.transaction_id WHERE MONTH(c.transaction.date) = MONTH(CURRENT_DATE()) AND YEAR(c.transaction.date) = YEAR(CURRENT_DATE())"
    Using cn As New SqlConnection("Your connection string"),
            cmd As New SqlCommand(sql, cn)
        RetVal = CDec(cmd.ExecuteScalar())
    End Using
    Return RetVal
End Function

【讨论】：

【解决方案2】：

似乎有一些代码是从其他东西遗留下来的。由于您只需要一个值，您可以使用ExecuteScalar，如下所示：

Dim sql = "
    SELECT SUM(a.qty_sold)
    FROM sales AS a
    INNER JOIN transaction_history AS c
    ON c.Id = a.transaction_id
    WHERE MONTH(c.transaction.date) = MONTH(CURRENT_DATE())
    AND YEAR(c.transaction.date) = YEAR(CURRENT_DATE())"

Dim currentMonthProductsSold = 0

Using conn As New SqlConnection("yourConnectionString"),
       cmd As New SqlCommand(sql, conn)

    conn.Open()
    currentMonthProductsSold = CInt(cmd.ExecuteScalar())

End Using

MessageBox.Show(currentMonthProductsSold)

【讨论】：