具有多个 ON OR 条件的 LINQ Left Join

Question

很抱歉，我在 LINQ 上有点薄弱，我总是在开始处理复杂的 LINQ 之前编写 SQL 查询。

我想问一下如何使用LEFT JOIN 和多个ON 条件使用OR 运算符 将此SQL 查询转换为LINQ。，

m.MerchandiseId 将在 ON 条件下使用两次

SELECT
    *
FROM
    Inbox AS i
    INNER JOIN [User] AS u ON i.FromUserId = u.UserId
    LEFT OUTER JOIN Merchandise AS m ON
        u.MerchandiseId = m.MerchandiseId
        OR
        i.ToMerchantId = m.MerchandiseId
WHERE
    i.ToCompanyId = 10
    OR
    i.FromCompanyId = 10



var message = (from i in db.Inbox
               join u in db.User on i.FromUserId equals u.UserId
               join m in db.Merchandise on u.MerchandiseId equals m.MerchandiseId //here I want to ON i.MerchantId = m.MerchandiseId, but it doesn't allow
                where i.ToCompanyId == user.CompanyId || i.FromCompanyId == user.CompanyId
                orderby i.CreatedAt descending
                group m.MerchandiseId by new { m.MerchandiseId, m.MerchandiseName } into grp
                select new
                {
                       MerchandiseId = grp.Key.MerchandiseId,
                       MerchandiseName = grp.Key.MerchandiseName,
                       InboxMessage = (from e in db.Inbox
                                        join us in db.User on e.FromUserId equals us.UserId
                                         join mer in db.Merchandise on us.MerchandiseId equals mer.MerchandiseId
                                          where mer.MerchandiseId == grp.Key.MerchandiseId
                                          orderby e.CreatedAt descending
                                          select e.InboxMessage).FirstOrDefault(),
                       CreatedAt = (from e in db.Inbox
                                    join us in db.User on e.FromUserId equals us.UserId
                                    join mer in db.Merchandise on us.MerchandiseId equals mer.MerchandiseId
                                     where mer.MerchandiseId == grp.Key.MerchandiseId
                                     orderby e.CreatedAt descending
                                     select e.CreatedAt).FirstOrDefault(),
                                       }).ToList();

我为它编写的底部 LINQ 查询。但是，我只能在 LINQ 中使用多个 ON 子句处理左连接。感谢有人会在这方面帮助我。谢谢！

Answer 1

我不相信 Linq 支持在多列中使用 OR 运算符，但也就是说，即使在 SQL 中，我也不会使用 OR，因为它使连接的意图不清楚，而且它也模糊了数据来自 - 如果每列有多个匹配项，也不清楚会发生什么。相反，我会在不同的列上两次 JOIN 并让投影子句处理它：

SELECT
    *
FROM
    Inbox
    INNER JOIN [User] AS u ON i.FromUserId = u.UserId
    LEFT OUTER JOIN Merchandise AS userMerchant ON u.MerchandiseId = userMerchant.MerchandiseId
    LEFT OUTER JOIN Merchandise AS inboxMerchant ON Inbox.ToMerchantId = inboxMerchant .MerchandizeId
WHERE
    Inbox.ToCompanyId = 10
    OR
    Inbox.FromCompanyId = 10

然后可以使用 LEFT OUTER JOIN 方法 (How to implement left join in JOIN Extension method) 将其转换为 Linq

请注意，如果您使用的是实体框架，那么您根本不需要担心这样做！只需使用Include：

var query = db.Inbox
    .Include( i => i.User )
    .Include( i => i.User.Merchandise )
    .Include  i => i.Merchandise )
    .Where( i => i.ToCompanyId = 10 || i.FromCompanyId == 10 );