將SQL查詢轉換為Linq左連接子句不正確

Question

我正在嘗試選擇Unlock表中不存在的MatchingObjects，我對此SQL查詢如下：

select a.* from MatchingObjects a 
    left join Unlocks b
        on a.ObjectCategoryId = b.ObjectCategoryId
    left join Members c
        on b.StudentId = c.Id
            and b.StudentId = @studentId
where  b.ObjectCategoryId is null
    and c.id is null
order by a.ObjectCategoryId

和一個LINQ查詢

var query = (from d in db.ObjectCategories
                     join a in db.MatchingObjects on d.Id equals a.ObjectCategoryId into grp3
                     join b in db.Unlocks
                         on d.Id equals b.ObjectCategoryId into grp1
                     from m in grp1.DefaultIfEmpty()
                     join c in db.Members
                         on m.StudentId equals c.Id into grp2
                     from n in grp2.DefaultIfEmpty()
                     where m.ObjectCategoryId == null
                     && n.Id == null
                     orderby d.Id).AsEnumerable()
                     ;

但是，LINQ查詢未顯示與SQL查詢相同的結果。 你們能告訴我在LINQ查詢中應該更改什么嗎？

這是模型：

Answer 1

更好的是，您可以使用以下工具：

SQL-> LINQ轉換器。

1. http://www.sqltolinq.com

2. http://www.linqpad.net/

Answer 2

試試這個

var query = from m in db.MatchingObjects.Where(w => w.ObjectCategoryId == null)
                            join u in db.Unlocks.Where(w => w.studentId == @studentId)
                                on m.ObjectCategoryId equals u.ObjectCategoryId into joinedU
                            from ju in joinedU.DefaultIfEmpty()
                            join m in db.Members.Where(w => w.id == null)
                                on ju.StudentId equals m.Id into joinedM
                            from jm in joinedM.DefaultIfEmpty()
                            select m;

但是您的要求有點奇怪。 您通過ObjectCategoryId進行聯接，並在Where子句中將ObjectCategoryId ==設置為空！！！

Answer 3

抱歉，Sql和LINQ查詢在表選擇方面有些不同。 我對此感到抱歉，因為我在發布問題時嘗試使用其他Linq，但是主要問題在於join子句

(from d in db.ObjectCategories
                     join a in db.MatchingObjects
                        on d.Id equals a.ObjectCategoryId into grp3
                     join b in db.Unlocks
                        on d.Id equals b.ObjectCategoryId into grp1
                     from m in grp1.DefaultIfEmpty()
                     join c in db.Members 
                        on m.StudentId equals studentId into grp2
                     from n in grp2.DefaultIfEmpty()
                     where m.ObjectCategoryId == null
                     where n.Id == null
                     orderby d.Id).AsEnumerable()


/* this is the correct one */
join c in db.Members 
on m.StudentId equals studentId into grp2

/* the below was the original incorrect join clause*/
join c in db.Members
on m.StudentId equals c.Id into grp2