LINQ雙左連接

Question

我有一個學生類，名稱，具有學生名稱和條目標題的blogEntries類以及具有任務名稱的blogAssignments類。

我想向所有學生和他們旁邊顯示他們創建的blogEntries，其標題與blogAssignments.assignmentName匹配。

我有以下LINQ查詢，但我無法揣摩出在blogAssignments加盟：

var results = 
       from person in HACCStudents 
       join bEntry in blogEntries on person.FullName equals bEntry.AuthorFullName 
       into studentBlogEntries 
       from studentBlogEntry in studentBlogEntries.DefaultIfEmpty() 
       orderby person.FullName 
       select new { PersonName = person.FullName, 
        BlogTitle=studentBlogEntry != null ?studentBlogEntry.Title : "" 


       }; 

Answer 1

編輯：

這裏是我寫的，我認爲一個測試用例滿足您的使用LINQ to Objects的要求：

public class Person 
{ 
    public string FullName; 
} 

public class BlogEntry 
{ 
    public string AuthorFullName; 
    public string Title; 
} 

public class BlogAssignment 
{ 
    public string AssignmentName; 
} 

public void GetStudentsWithAssignments() 
{ 
    var students = new List 
    { 
     new Person { FullName = "Bob Doe" }, 
     new Person { FullName = "Steve Carrell" }, 
     new Person { FullName = "John West" }, 
     new Person { FullName = "Alice Croft" }, 
     new Person { FullName = "Al Bean" } 
    }; 

    var entries = new List 
    { 
     new BlogEntry { AuthorFullName = "Steve Carrell", Title = "100 ways to skin a cat" }, 
     new BlogEntry { AuthorFullName = "Steve Carrell", Title = "LINQ sux arze" }, 
     new BlogEntry { AuthorFullName = "John West", Title = "Salmon bake 2007" }, 
     new BlogEntry { AuthorFullName = "John West", Title = "Make my day" }, 
     new BlogEntry { AuthorFullName = "Al Bean", Title = "An inconvenient tooth" } 
    }; 

    var assignments = new List 
    { 
     new BlogAssignment { AssignmentName = "Salmon bake 2007" }, 
     new BlogAssignment { AssignmentName = "LINQ sux arze" }, 
     new BlogAssignment { AssignmentName = "Make my day" } 
    }; 

    var subquery = 
     from e in entries 
     join a in assignments on e.Title equals a.AssignmentName 
     select e; 

    var results = 
     from person in students 
     join entry in subquery on person.FullName equals entry.AuthorFullName into personEntries 
     from personEntry in personEntries.DefaultIfEmpty() 
     orderby person.FullName 
     select new 
     { 
      PersonName = person.FullName, 
      BlogTitle = personEntry == null ? "" : personEntry.Title 
     }; 
} 

更棘手的是你想顯示所有學生，但只有左連接博客與博客分配相匹配的條目，這是一個內部連接。

這就是爲什麼我添加了進行內連接的子查詢，然後在主查詢中使用join..into語法將連接添加到子查詢中，以獲得左外連接語義。

您可以一口氣做到這一切，但我發現通過將子查詢分解爲單獨的聲明更容易閱讀LINQ查詢。