这是相当于查询到 LEFT OUTER 加入？

  //假设int类型的，我有一个名为参数'invoiceId
从C在SupportCases
让发票= c.Invoices.FirstOrDefault（I =＆GT; i.Id == invoiceId）
其中，（invoiceId == 0 ||发票！= NULL）
选择新
{
      n = c.Id
      ，InvoiceId =发票== NULL？ 0：invoice.Id
}
 

解决方案

不太 - 因为每次左的行中的左外连接将匹配0-N右行（第二个表），其中， -as你只有0-1匹配。做一个左外连接，则需要的SelectMany 和 DefaultIfEmpty ，例如：

  VAR的查询=从C在db.Customers
            加盟澳中db.Orders
               在c.CustomerID等于o.CustomerID到SR
            从x在sr.DefaultIfEmpty（）
            选择新{
               客户id = c.CustomerID，联系人姓名= c.ContactName，
               订单ID = x.OrderID == NULL？ -1：x.OrderID};
 

（or通过扩展方法）

Is this query equivalent to a LEFT OUTER join?

//assuming that I have a parameter named 'invoiceId' of type int
from c in SupportCases
let invoice = c.Invoices.FirstOrDefault(i=> i.Id == invoiceId)
where (invoiceId == 0 || invoice != null)    
select new 
{
      Id = c.Id
      , InvoiceId = invoice == null ? 0 : invoice.Id
}

Not quite - since each "left" row in a left-outer-join will match 0-n "right" rows (in the second table), where-as yours matches only 0-1. To do a left outer join, you need SelectMany and DefaultIfEmpty, for example:

var query = from c in db.Customers
            join o in db.Orders
               on c.CustomerID equals o.CustomerID into sr
            from x in sr.DefaultIfEmpty()
            select new {
               CustomerID= c.CustomerID, ContactName=c.ContactName,
               OrderID = x.OrderID == null ? -1 : x.OrderID};   

(or via the extension methods)