这个查询相当于LEFT OUTER个连接吗？
//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
}
不完全是-因为左-外连接中的每个"左"行都将匹配0-n个"右"行(在第二个表中)，其中-as您的只匹配0-1.要执行左外部联接，您需要SelectMany和DefaultIfEmpty，例如:
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 == null ? -1 : x.OrderID };