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如何获取Linq查询的outerjoin

linq-to-entities linq

Ralph Shillington · 技术社区 · 15 年前

给出这个linq查询

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

在这种情况下,我需要客户和订单之间的外部连接。我该怎么在林肯表达呢?我认为TSQL的近似值是

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

2 回复 | 直到 15 年前

Community CDub 8 年前

要获取外部联接,可以使用DefaultIfEmpty。请看这个问题: 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,您可以执行以下操作:

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

我相信它们都生成相同的SQL。

codingbadger 15 年前

你必须使用 DefaultIfEmpty

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

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