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Question

我正在学习 Linq2SQL，我对左外连接有疑问。在下面的示例中，我相信我正在对问题 table 执行左外连接到最喜欢的问题 table。但是我不相信我的 where 子句是正确的。因此，如果我对两个 table 执行遗漏连接，我应该如何适当地设置 where 子句？

var myResults = from quest in context.MyQuestions
                join favQuest in context.MyFavoriteQuestions on quest.UserFavoriteQuestionId equals favQuest.UserFavoriteQuestionId
                join specialQuest in context.Questions on favQuest.QuestionId equals specialQuest.QuestionId into joinedQuestions
                from specialQuest in joinedQuestions.DefaultIfEmpty()
                where (quest.UserId == userId) &&
                                    ( specialQuest.Id == paramId && (!specialQuest.IsBlue || (specialQuest.IsBlue && canViewBlueQuestion)) &&
                                      (!specialQuest.IsRed || (specialQuest.IsRed && canViewRedQuestion))
                                    )
                              select quest;

Answer 1

对于 LINQ to SQL 上下文，建议这样编写左外连接，因为它实际上会生成 SQL LEFT JOIN:

var myResults = from question in context.MyQuestions
from favoriteQuestion in context.MyFavoriteQuestions
    .Where(fc => fc.UserFavoriteQuestionId == question.UserFavoriteQuestionId)
    .DefaultIfEmpty()

还建议（以提高易读性）分隔不相关的（和 ANDed）where 从句：

var myResults = from question in context.MyQuestions
                where question.UserId == userId
                from favoriteQuestion in context.MyFavoriteQuestions
                    .Where(fc => fc.UserFavoriteQuestionId == question.UserFavoriteQuestionId)
                    .DefaultIfEmpty()
                from specialQuestion in context.Questions
                    .Where(sc => sc.QuestionId == favoriteQuestion.QuestionId)
                    .DefaultIfEmpty()
                where specialQuestion.Id == paramId
                where !specialQuestion.IsBlue || (specialQuestion.IsBlue && canViewBlueQuestion)
                where !specialQuestion.IsRed || (specialQuestion.IsRed && canViewRedQuestion)
                select question;

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