许多SQL连接到LINQ的外连接

Question

尝试用许多外连接重写SQL查询到Linq很困难。

这是查询：

select on1.diskpath as d1, 
on2.diskpath as d2, 
of1.diskpath as d3, 
of2.diskpath as d4, 
on1.disknaam as n1, 
on2.disknaam as n2, 
of1.disknaam as n3, 
of2.disknaam as n4 
from tblstoragelocation 
left join tblstoragedisks on1 on online1=on1.id 
left join tblstoragedisks on2 on online2=on2.id 
left join tblstoragedisks of1 on offline1=of1.id 
left join tblstoragedisks of2 on offline2=of2.id where md5='xxx'"; 

我尝试了很多东西，这是其中之一，但它提供了许多错误：（LOC已经宣布，类型推断在群组加入失败，ONLINE2找不到）

from loc in fdc.tblStoragelocations 
join _on1 in fdc.tblStoragedisks on loc.online1 equals _on1.id into on1 
from _on2 in fdc.tblStoragedisks on loc.online2 equals _on2.id into on2 

因此，如何写多个左外连接到LINQ？

Answer 1

左外连接是通过.DefaultIfEmpty（）方法实现的。

var q = 
    from iter_1 in collection_1 
    join iter_2 in collection_2 on iter_1 equals iter_2 into join_1 
    from iter_2 in join_1.DefaultIfEmpty() 
    join iter_3 in collection_3 on iter_2 equals iter_3 into join_2 
    from iter_3 in join_2.DefaultIfEmpty() 
    ... 
    join iter_n in collection_n on iter_n_1 equals iter_n into join_n_1 
    from iter_n in join_n_1.DefaultIfEmpty() 
    select join_n_1;