.net - LINQ to SQL 左外连接

Question

此查询是否等同于 LEFT OUTER 联接？

//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

您不需要 into 语句:

var query = 
    from customer in dc.Customers
    from order in dc.Orders
         .Where(o => customer.CustomerId == o.CustomerId)
         .DefaultIfEmpty()
    select new { Customer = customer, Order = order } 
    //Order will be null if the left join is null

是的，上面的查询确实创建了一个 LEFT OUTER 连接。

链接到处理多个左连接的类似问题: Linq to Sql: Multiple left outer joins