sql - 在 VB.net 中使用 SQL 查询读取平均值

Question

我需要在 vb.net 中运行 SQL 查询来找到我的应用程序的平均评分。我认为这是执行此操作的正确 SQL：

SELECT AVG(Rating) FROM Feedback;

唯一的问题是我不确定如何正确运行此查询。我有一个类似的查询，我已将其更改为以下内容（它不起作用），我只是想知道您是否可以帮助调整它以使其真正起作用？

    Dim conn As New MySqlConnection(DBCred)
    Dim sqlquery As String = "SELECT AVG(Rating) FROM Feedback;"
    Dim cmd As MySqlCommand
    Dim AVGRating As String
    conn.Close()
    cmd = New MySqlCommand(sqlquery, conn)
    conn.Open()
    Try
        AVGRating = cmd.r
    Finally
        cmd.Connection.Close()

    End Try
    conn.Close()

谢谢克里斯

Answer 1

试试这个。

    Dim AVGRating As String = String.Empty
    Using xConn As New MySqlConnection(DBCred)
        Using xComm As New MySqlCommand()
            With xComm
                .Connection = xConn
                .CommandType = CommandType.Text
                .CommandText = "SELECT AVG(Rating) FROM Feedback WHERE Feedback.Rating > 0"
            End With
            Try
                xConn.Open()
                AVGRating = CStr(xComm.ExecuteScalar)
            Catch ex As Exception
                MsgBox(ex.Message.ToString)
            Finally
                xConn.Close()
            End Try
        End Using
    End Using

链接：SqlCommand 对象中的 ADO.NET ExecuteScalar

Answer 2

您需要执行命令 - 在这种情况下，使用ExecuteScalar

AVGRating = cmd.ExecuteScalar().ToString()