Question

如果它们存在，我想从表a中选择值，但如果它们不存在则从b中选择值。在正常的查询中，我可以做leftjoin，但我不确定它是如何工作的

public static IEnumerable<dynamic> Foo(string somevalue, ref string errMessage)
{
    try 
    {
    var query=     (from b in context.Tableb
                    join a in context.Tablea on b.value1 equals a.value1
                    where b.value1 == "somevalue"
                    select new Selectclass()
                    {
                        value1 = b.value1,
                        value2 = a.value2 ?? b.value2,
                        value3 = a.value3 ?? b.value3,
                        value4 = a.value4 ?? b.value4,
                        value5 = a.value5 ?? b.value5,
                        value6 = a.value6.ToString("dd-MM-yyyy"), //value7 is a DateTime in my Tables
                        value7 = a.value7 
                    }).ToArray();

    return query;
    }
    catch (Exception ex)
        {
            errMessage += (string.IsNullOrEmpty(errMessage) ? "" : Environment.NewLine) +
                          (string.IsNullOrEmpty(ex.Message) ? "" : ex.Message) +
                          (ex.InnerException != null ? Environment.NewLine + ex.InnerException.Message : "");
            MessageBox.Show(errMessage);
            return null;
        }
}

public class SelectClass
{
    string value1 = String.Empty;
    string value2 = String.Empty;
    string value3 = String.Empty;
    string value4 = String.Empty;
    string value5 = String.Empty;
    string value6 = String.Empty;
    string value7 = String.Empty;
}

我的Tablea包含与Tableb相同的值，另外它包含Value6和Value7，我希望将其作为String.Empty返回，如果Tablea中不存在“somevalue”，则返回null

我回来的只是一个空的IEnum。有什么帮助吗？

Answer 1

使用DefaultIfEmpty模拟LINQ中的左连接：

var query=     (from b in context.Tableb
                where b.value1 == "somevalue"
                join a in context.Tablea on b.value1 equals a.value1 into ja
                from a in ja.DefaultIfEmpty()
                select new Selectclass()
                {
                    value1 = b.value1,
                    value2 = a?.value2 ?? b.value2,
                    value3 = a?.value3 ?? b.value3,
                    value4 = a?.value4 ?? b.value4,
                    value5 = a?.value5 ?? b.value5,
                    value6 = a?.value6.ToString("dd-MM-yyyy"), //value7 is a DateTime in my Tables
                    value7 = a?.value7 
                }).ToArray();

LinqConnect连接抛出异常

1 个答案: