无法获取数据表中列的总和

Question

我正在尝试编写一种方法，该方法返回两个日期之间条目的列总和。然而，无论如何它都会返回DBnull。当查询更简单时，我还有其他类似的方法（没有 SUM，简单的select * where statement）。 performQuery() 只是帮助方法，它返回一个填充的datatable 和查询结果。

    public static int getBookedQuantity(int shopID, int bikeID, DateTime datetime)
    {
        string query = "SELECT sum(quantity) as \"quantity\" FROM booking WHERE bikeID=@bikeID AND shopID = @shopID AND starttime >= @Datetime AND endtime < @Datetime";
        SqlParameter param1 = new SqlParameter("@bikeID", bikeID);
        SqlParameter param2 = new SqlParameter("@shopID", shopID);
        SqlParameter param3 = new SqlParameter("@Datetime", SqlDbType.DateTime);
        param3.Value = datetime;
        DataTable bookingData = performQuery(query, param1, param2, param3);
        DataRow[] row = bookingData.Select();
        int totalBooked = 0;
        if ((row.Count()) > 0 && (bookingData != null)) // if there are rows returned
            totalBooked = Convert.ToInt32(row[0]["quantity"]);
        return totalBooked;
    }

提前谢谢你！

Answer 1

尝试将查询修改为

string query = "SELECT sum(quantity) as 'quantity' FROM booking WHERE bikeID=@bikeID AND shopID = @shopID AND starttime >= @Datetime AND endtime < @Datetime";

Answer 2

DataTable 不是获取总和的正确方法。要解决您的问题，您必须这样做：

var sum = Convert.ToInt32(bookingData.Rows[0]["quantity"]);
return sum;

但是，执行此命令的正确方法是：

var statement = "SELECT COUNT(*) ...";
var command = new SqlCommand(statement, connection);
command.AddParam(param1);
command.AddParam(param2);
command.AddParam(param3);
var sum = Convert.ToInt32(command.ExecuteScalar());
return sum;

有关标量的信息，请参阅 MSDN 文档：https://msdn.microsoft.com/it-it/library/system.data.sqlclient.sqlcommand.executescalar(v=vs.110).aspx

希望这能有所帮助。