Linq To SQL加入

Question

我在學習Linq2SQL，並且在左外連接上有一個問題。 在我下面的示例中，我相信我正在將問題表上的左外連接執行到favoritequestions表。但我不相信我的where子句是正確的。 因此，如果我在兩個表上執行一個左出連接，我應該如何適當地設置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

對於LINQ到SQL上下文，建議寫左外連接同樣地，作爲實際上產生一個SQL LEFT JOIN：

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

還建議（提高易讀性），以獨立無關的（和AND ED）where子句：

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;