Question

您好我正在尝试将以下SQL语句转换为其LINQ等价物，因为我真的是.net（编码一天），我已经被困在这几个小时了。

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这是模型

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

我非常感谢任何人的帮助。

修改

上下文类

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; } 
}

Answer 1

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 2

假设您的上下文被称为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 3

最好通过连接来执行此操作。

论点是：如果用户借阅了大量的书籍，或者数据中存在错误，那么您的子查询可能会返回大量ID，并且长列表中的SQL“IN”子句可能会变得非常慢。 / p>

使用联接：

SQL查询：

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

Linq声明：

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

Answer 4

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 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

SQL到IQueryable LINQ

5 个答案: