Question

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 } 

Answer 1

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 == null ? -1 : x.OrderID };    

(or via the extension methods)