SQL 到 IQueryable LINQ

Question

您好，我正在尝试将以下 SQL 语句转换为其等效的 LINQ 语句，由于我对 .net 非常陌生（编码一天），我已经被困在这几个小时了。

SELECT * 
 FROM Books 
 WHERE BookID IN (SELECT BookID 
              FROM Borrows 
              WHERE UserID = 2) 

这是模型

public class LibUser
{       
    [Key]
    public int UserID { get; set; }
    [Required, StringLength(50), Display(Name = "First Name")]
    public string UserFirstName { get; set; }
    [Required, StringLength(50), Display(Name = "Last Name")]
    public string UserLastName { get; set; }
    [Required, StringLength(10000), Display(Name = "Residence"), DataType(DataType.MultilineText)]
    public string Adress { get; set; }
}

public class Book { 
    [Key]
    public int BookID { get; set; }
    public string Title { get; set; }
    public string Author { get; set; }
    public DateTime Published{ get; set; }
}

public class Borrowed {
    [Key]
    public int BorrowID { get; set; }
    public int UserID { get; set; }
    public int BookID { get; set; } 
}

非常感谢任何人的帮助。

编辑

上下文类

public class LibraryContext : DbContext
{

          public LibraryContext()
        : base("libromatic")
    {
    }

    public DbSet<LibUser> LibUsers { get; set; }
    public DbSet<Book> Books { get; set; }
    public DbSet<Borrowed> Borrows { get; set; }

}

Answer 1

假设您的上下文称为db，您可以执行以下查询

var borrowedBooksForUser = db.Books
     .Where(b => db.Borrowed.Any(x => x.UserID == 2 && x.BookID == b.BookID));

Answer 2

最好使用连接来执行此操作。

论据是：如果用户借了大量书籍，或者数据有错误，那么您的子查询可能会返回大量 ID，而长列表上的 SQL 'IN' 子句会变得非常慢。

使用连接：

SQL 查询：

SELECT Books.* FROM Books 
  JOIN Borrows ON Borrows.BookID = Books.BookID
  WHERE Borrows.UserID = 2 

Linq 语句：

var allBooksBorrowedByUser2 = db.Borrowed
                .Where(borrow => borrow.UserID == 2)
                .Join(db.Books,
                      borrow => borrow.BookID,
                      book => book.BookID,
                      (borrow, book) => book);

Answer 3

导航会让一切变得更简单。

public class Borrowed {
    [Key]
    public int BorrowID { get; set; }
    public int UserID { get; set; }
    public int BookID { get; set; } 

    // Navigation Properties
    public virtual LibUser User { get; set; }
    public virtual Book Book { get; set; }
}

Borrows.Where(borrow => borrow.UserId == 2)
       .Select(borrow => borrow.Book);

Answer 4

你可以这样做：

var Lnq = new LinqDataContext();
var borrowId = Lnq.Borrowed.Where(a => a.UserID == 2).Select(a => a.BookID).ToList();

var bookQuery = Lnq.Books.Where(a => borrowId.Contains(a.BookID))
                         .Select(a => a.YourColumn);

Answer 5

试试这个，

  var getResult=from b in db.Books
  join bo in db.Borrows on b.BookID=bo.BookID
  where bo.UserID=2