LINQ - 与 Group by 多连接并获得平均值

Question

我正在尝试在 LINQ 中编写此选择，但我无法成功修复它很长时间。我也试过LINQ - join with Group By and get average，但它在我的代码中不起作用。很明显我错了。

SQL：

SELECT name_type, AVG(t.price) as avgPrice FROM type tp
JOIN location l ON l.ID_type = tp.ID 
JOIN event e ON e.ID_location = l.ID
JOIN ticket t ON t.ID_event = e.ID
GROUP BY tp.name_type

LINQ：

var q3 = from l in db.location
join tp in db.type on l.ID_type equals tp.ID
join e in db.event on l.ID equals u.ID_location
join t in db.ticket on e.ID equals t.ID_event 
group tp by new {Type_name = tp.type_name} into grp
select new
{
     Type_name = grp.Key.type_name,
     avgPrice = grp.Average( x => x.ticket.price)
};

Answer 1

有几个问题：

1. 第二次连接有错误——我认为u.ID_location 必须是e.ID_location。
2. 我认为您分组的实体有误，请尝试按 t 而不是 tp 分组。
3. group by 中不需要匿名类型。

试试这个：

var results = 
     from l in db.location
     join tp in db.type on l.ID_type equals tp.ID
     join e in db.event on l.ID equals e.ID_location
     join t in db.ticket on e.ID equals t.ID_event 
     group t by new tp.type_name into grp
     select new
     {
          Type_name = grp.Key,
          avgPrice = grp.Average(x => x.price)
     };

如果您碰巧在实体之间设置了导航属性，这会容易得多。很难说这些实体是如何应该相关的，但我认为这样的事情会起作用：

// average ticket price per location type
var results = 
    from t in db.ticket
    group t by t.event.location.type.type_name into g
    select new
    {
         Type_name = g.Key,
         avgPrice = g.Average(x => x.price)
    }; 

或者用流利的语法：

var results = db.ticket.GroupBy(t => t.event.location.type.type_name)
                       .Select(g => new 
                               { 
                                   Type_name = g.Key, 
                                   avgPrice = g.Average(x => x.price) 
                               });