使用空值查询表 [英] Query on table with nulls

问题描述

我对一个表中有一个查询，其中列中允许空值。然后，该方法将查询结果作为数组返回以绘制图表。 chart命令采用Windows窗体，数据库查询位于类中。

我认为查询行

I have a query on a table where nulls are allowed in the columns. The method then returns the query results as an array to plot a chart. The chart command is in a windows form and the database query is in a class.
I thought the query line

where P!=null

排除了空值，但我仍然收到错误

excluded nulls, but I still get an error

"can't convert double[]? to double"

必须有一个简单的修复，但我在网上找不到任何东西或MSDN 。[ ^ ]



我正在圈子里...感谢您的帮助。



我尝试了什么：

There has to be a simple fix but I can't find anything on the net or MSDN .[^]

I'm going in circles...thanks for the help.

What I have tried:

//a command in a windows form to plot a chart
private void cmdCHART_Click(object sender, EventArgs e)
{

    nmCHART.clsCHART oCHART = new nmCHART.clsCHART();
            
    double[] Y = new double[21];
    Y = oCHART.GET_Y_SERIES("CONDOR");
}


//code in a class to retrieve data from a table
namespace nmCHART
{
    public class clsCHART
    {
        public double?[] GET_Y_SERIES(string GREEK)        
        {

            var qryY = (from P in Globals.DATA.PAYOFF_EAVs
                       where P.GREEK == GREEK
                       where P != null
                       orderby P.DP_NO
                       select P.DATA).ToArray();

            return qryY;
        }
    }
}

推荐答案

您已声明方法返回 double 但是试图返回 double？[] 。



编辑[MTH]：

You've declared the method to return a double but are trying to return double?[].

Edit [MTH]:

//a command in a windows form to plot a chart
private void cmdCHART_Click(object sender, EventArgs e)
{
    // Does this really need to be created new each time it is used?
    // If nmCHART carries some important state, then it should be a singleton
    // Otherwise it should be a static class
    nmCHART.clsCHART oCHART = new nmCHART.clsCHART();
            
    double[] Y = oCHART.GET_Y_SERIES("CONDOR");
}
 

//code in a class to retrieve data from a table
namespace nmCHART
{
    public class clsCHART
    {
        public double[] GET_Y_SERIES(string GREEK)        
        {
            var qryY = (from P in Globals.DATA.PAYOFF_EAVs
                       where P.GREEK == GREEK
                             && P.DATA != null // as Sascha Lefèvre noted in Solution 2
                       orderby P.DP_NO
                       select P.DATA).OfType<double>().ToArray();
 
            return qryY;
        }
    }
}

.OfType< double>（）从 double？将所有值转换为 double ，以便调用者可以使用它而不必担心 null s。

此外， .OfType< double>（）将过滤掉 null 值，因此查询查询时不需要 null ，但是，如果这实际上是数据库查询， null 检查将在服务器端执行，因此可能更有效。

The .OfType<double>() casts all of the values to double from double? so the caller can use it without having to be concerned about nulls.
Also, the .OfType<double>() will filter out the null values, so the null check in the query is not necessary, but, if this is actually a database query, the null check would be executed on the server side, so it could be more efficient.

你选择 P.DATA



所以你应该检查 P.DATA 因为不是空的，不是 P ：

You select P.DATA

So you should check P.DATA for not being null, not P:

var qryY = (from P in Globals.DATA.PAYOFF_EAVs
            where P.GREEK == GREEK
            && P.DATA != null
            orderby P.DP_NO
            select P.DATA
           ).ToArray();

这是另一个解决方案的帮助其他

here is another solution with the help of others

double[] Y = new double[21];
            Y = oCHART.GET_Y_SERIES("CONDOR").Where(d => d.HasValue).Cast<double>().ToArray();

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