Linq - 按多个表分组

Question

使用 Linq to Sql 我如何对以下 2 个表进行分组。

订单表：

CustomerID | Name   |Date            
1          | order1 | 2010-01-01  
2          | order2 | 2010-01-01
2          | order3 | 2010-04-01

来电表：

CustomerID | Name   |Date            
1          | call1 | 2010-01-01  
3          | call2 | 2010-06-01
2          | call3 | 2010-05-01

我想按日期对两张表进行分组，结果：

Date       | Orders | Calls
2010-01-01 | 2      | 1
2010-04-01 | 1      | 0
2010-05-01 | 0      | 1
2010-06-01 | 0      | 1

我知道如何对单个表进行分组，

from o in Orders        
group o by o.Date.Date into og
select new {Date = og.Key,Orders= og.Count()};

如何将两者分组？ 谢谢！

Answer 1

由于这两个表似乎具有相似的结构，我建议将它们投影到等效的形式中，然后对这两个集合的串联进行分组。

var orders = from o in Orders 
            select new { IsOrder = true, o.Date };
var calls = from c in Calls 
            select new { IsOrder = false, c.Date };

var result = from x in orders.Concat(calls)        
            group x by x.Date into og
            select new {Date = og.Key, Orders= og.Count(o=>o.IsOrder), Calls = og.Count(c=>!c.IsTrue)};

由于 Linq2Sql 的惰性特性，这实际上可能会简化为单个查询。为了提高性能，我会确保这不是来自地狱的查询。

Answer 2

可以使用 Union 方法：

    var result =
        (from c in Calls group c by c.Date into cg select new {Date = cg.Key, Calls = cg.Count(), Orders = 0})
        .Union(from o in Orders group o by o.Date into og select new {Date = og.Key, Calls = 0, Orders = og.Count()})
        .GroupBy(x => x.Date)
        .Select(g => new {Date = g.Key, Calls = g.Max(r => r.Calls), Orders = g.Max(r => r.Orders)});

    foreach (var row in result)
    {
        Trace.WriteLine(row);
    }

这与您编写的 SQL 非常相似（两个表的并集，然后是外部查询以将结果合并到一行中）