如何解决如何在C#中使用LINQ应用右外连接?
我面临在LINQ中应用Right Outer Join的问题。我正在尝试通过从SQL查询转换来在LINQ中构建类似的查询。
我正确的SQL查询
select *
from Table1 t1
inner join Table2 t2 on t1.Id = t2.FID and t1.CID = 20
right outer join Table3 t3 on t1.GID = t3.GID
var result = from t1 in Table1
join t2 in Table2
on t1.ID equals t2.FID into gr1
where t1.CID = 20
join t3 in Table3
on t1.GID equals t3.GID into gr2
from t3 in gr2.DefaultIfEmpty()
select new Details (){
}
return result.ToList();
}
我的LINQ查询无法像SQL查询那样正常工作。
解决方法
这也在这里讨论 how to make a right join using LINQ to SQL & C#
var RightJoin = from adds in dc.EmpresaAddendas
join cats in CatAddendas
on adds.IDAddenda equals cats.IDAddenda into joined
from cats in joined.DefaultIfEmpty()
select new
{
Id = cats.IDAddenda,Description = cats.Descripcion
};
,
尝试一下:
var result = from t1 in Table1
join t2 in Table2
on new { id = t1.ID; cid = t1.CID } equals new { id = t2.FID: cid = 20} into gr1
join t3 in Table3
on t1.GID equals t3.GID into gr2
from t123 in gr2.DefaultIfEmpty()
select new Details (){
}
return result.ToList();
}
,
您会发现,大多数LINQ提供程序都不提供对right join
的支持。 DefaultIfEmpty转换为LEFT join
。结果,您需要将查询的逻辑稍微翻转一下,以将右联接变成左联接。
就实际语法而言,我已经成功采用了更多的ANSI-82语法,而不必担心into temp2 from temp3 in temp2
语法。类似于:
var result =
from t3 in Table3
from t1 in Table1.Where(temp1 => temp1.GID == t3.GID).DefaultIfEmpty()
join t2 in Table2 on t1ID equals t2.FID
where t1.CID == 20
select new Details{};
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