如何獲得LINQ查詢

Question

鑑於這種LINQ查詢

from c in context.Customers 
from o in c.Orders 
where c.City == "MyCity" || o.ShipTo == "MyCity" 
select c 

如果客戶的城市是「MyCity」，但沒有任何訂單的查詢將不會返回任何行外連接。這是因爲客戶和訂單之間隱含的內部聯接。如何選擇「MyCity」城市的客戶或訂單發貨到「MyCity

在這種情況下，我需要一個客戶和訂單之間的外部連接，我如何在Linq中表達？我認爲大致的TSQL是

select customers.* 
from customers 
left join orders on customers.id = orders.customerid 
where customers.city = 'MyCity' or orders.ShipTo = 'MyCity' 

Answer 1

爲了得到一個外連接，您可以使用DefaultIfEmpty看到這個問題：Linq to Sql: Multiple left outer joins

from c in context.Customers 
from o in context.Orders 
    .Where(a => a.customerid == c.id) 
    .DefaultIfEmpty() 
where c.City == "MyCity" || o.ShipTo == "MyCity" 
select c 

或者，你可以這樣做：

from c in context.Customers 
join o in context.Orders on c.id equals o.customerid into g 
from x in g.DefaultIfEmpty() 
where c.City == "MyCity" || x.ShipTo == "MyCity" 
select c 

我相信他們都生成相同的SQL。

Answer 2

你必須使用DefaultIfEmpty

我認爲像這樣的工作

var result = from c in context.Customers 
    join o in c.Orders on c.CustomerId equals o.CustomerId into lj 
    from or in lj.DefaultIfEmpty() 
    where c.City == "MyCity" || or.ShipTo == "MyCity" 
    select c