c# - 在 Linq 中子查询的有效方法

Question

这是我的 linq 代码:

BOOK entity = db.BOOKS
             .Where(s => s.ID == (from p in db.LIBRARY
                                  from b in db.BOOKS
                                  where (p.ID == 123) && (p.idpage == b.idpage)
                                  select b.fields));

我的实际 oracle 代码是:

SELECT DISTINCT BOOKS.ID 
FROM LIBRARY,BOOKS 
WHERE LIBRARY.ID = 123 AND LIBRARY.ID = BOOKS.ID

但是它在 s.ID 中显示错误......

Delegate 'System.Func Project.Models.BOOKS,int,bool' does not take 1 arguments

为什么会这样？有任何解决方法吗？

Answer 1

您的 SQL 使用联接，因此您可以在 LINQ 中执行相同的操作。这些方法中的任何一种都足够了:

// join
var query = (from b in db.BOOKS
            join p in db.LIBRARY on b.IdPage equals p.IdPage 
            where p.ID == 123
            select b.Id).Distinct();

// 2 from statements (SelectMany) can also be used as a join
var query = (from b in db.BOOKS
            from p in db.LIBRARY
            where p.ID == 123 && b.IdPage == p.IdPage 
            select b.Id).Distinct();

// fluent syntax
var query = db.BOOKS
              .Where(b => db.LIBRARY.Any(p =>
                  p.ID == 123 && b.IdPage == p.IdPage))
              .Select(b => b.Id)
              .Distinct();