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from a in db.A
join b in db.B on new {C=a.A,D=a.B} equals new {C=b.A,D=b.B} into j1
from b in j1.DefaultIfEmpyt()
join c in db.C on a.C equals c.C into j2
from c in j2.DefaultIfEmpty
select ……
from a in db.A
join b in db.B on new {C=a.A,D=a.B} equals new {C=b.A,D=b.B}
补充是匿名类的属性SELECT * FROM dbo.[Order] AS o INNER JOIN dbo.OrderDetail AS od
ON o.Id=od.OrderId LEFT JOIN dbo.Works AS wr
ON od.WorksId=wr.Id
from a in db.A
join b in db.B on new {a.A,a.B} equals new {b.A,b.B}
属性大小写名字要一样