Question

假设我有一个“数据库”定义为：

// Baked goods vendors
   var vendor = new[] {
    new { ID = 1, Baker = "Pies R Us", StMnemon = "NY", Items = 8, Rating = 9 },
    new { ID = 2, Baker = "Mikes Muffins", StMnemon = "CA", Items = 5, Rating = 9 },
    new { ID = 3, Baker = "Best Bakers", StMnemon = "FL", Items = 2, Rating = 5 },
    new { ID = 4, Baker = "Marys Baked Treats", StMnemon = "NY", Items = 8, Rating = 7 },
    new { ID = 5, Baker = "Cool Cakes", StMnemon = "NY", Items = 4, Rating = 9 },
    new { ID = 6, Baker = "Pie Heaven", StMnemon = "CA", Items = 12, Rating = 9 },
    new { ID = 7, Baker = "Cakes N More", StMnemon = "GA", Items = 6, Rating = 8 },
    new { ID = 8, Baker = "Dream Desserts", StMnemon = "FL", Items = 2, Rating = 7 }
};

// Locations
var location = new[] {
    new {ID= 1, State = "New York", Mnemonic = "NY"},
    new {ID= 2, State = "Massachusetts", Mnemonic = "MA"},
    new {ID= 3, State = "Ohio", Mnemonic = "OH"},
    new {ID= 4, State = "California", Mnemonic = "CA"},
    new {ID= 5, State = "Florida", Mnemonic = "FL"},
    new {ID= 6, State = "Texas", Mnemonic = "TX"},
    new {ID= 7, State = "Georgia", Mnemonic = "GA" }
};

我想构建一个等同于SQL查询的查询：

SELECT   State, Rating, SUM(Items) AS 'Kinds'
FROM     vendor, location
WHERE    vendor.StMnemon = location.Mnemonic
GROUP BY State, Rating

此查询中有两件事情是：

GROUP BY涉及多个表格，
结果包含未出现在分组标准中的列的总和。

我在grouping by multiple tables和summing columns not in the group-by的帖子中看到了解决方案。问题是两者结合并不真正复制关系查询。

我尝试使用以下代码在LINQ中复制它：

var query = from v in vendor
           join l in location
           on v.StMnemon equals l.Mnemonic
           orderby v.Rating ascending, l.State
           select new { v, l };

var result = from q in query
           group q by new { 
               s = q.l.State, 
               r = q.v.Rating
/* ==> */    , i = q.v.Items
           } into grp
        select new 
        {
            State = grp.Key.s,
            Rating = grp.Key.r
/* ==> */ , Kinds = grp.Sum(k => grp.Key.i)
        };

这导致：

=================================
State           Rating  Kinds
Florida         5       2
Florida         7       2
New York        7       8
Georgia         8       6
California      9       5
California      9       12
New York        9       8
New York        9       4
=================================

然而，上面给出的SQL查询给出了这个结果：

=========================
State       Rating  Kinds
Florida     5       2
Florida     7       2
New York    7       8
Georgia     8       6
California  9       17
New York    9       12
=========================

差异是因为除了分组标准之外，似乎没有地方可以添加其他列，这当然会更改分组结果。注释掉 / * ==＆gt;所指示的两行。上面代码中的* / 注释将提供与SQL结果相同的分组，但当然会删除我想要包含的求和字段。

我们如何在LINQ中对多个表进行分组并包含其他条件而不更改分组结果？

Answer 1

这样的东西似乎与SQL查询返回相同：

var result = from v in vendor
             from l in location
             where l.Mnemonic == v.StMnemon
             group v by new { l.State, v.Rating } into grp
             orderby grp.Key.Rating ascending, grp.Key.State
             select new {State = grp.Key.State, Rating = grp.Key.Rating, Kinds = grp.Sum(p=>p.Items)};

foreach (var item in result)
        Console.WriteLine("{0}\t{1}\t{2}", item.State, item.Rating, item.Kinds);

Answer 2

您可以在群组外进行汇总：

var query = from v in vendor
           join l in location
           on v.StMnemon equals l.Mnemonic
           orderby v.Rating ascending, l.State
           select new { v, l };

var result = from q in query
           group q by new { 
               s = q.l.State, 
               r = q.v.Rating
           } into grp
        select new 
        {
            State = grp.Key.s,
            Rating = grp.Key.r,
            Kinds = grp.Sum(g => g.Items)
        };

分组有点棘手 - 它返回一个IGrouping，它有一个属性 - Key。该分组中的实际项目由GetEnumerator()函数返回，该函数允许您将该组视为这些项的集合，这意味着您可以对该组中的项进行聚合。

LINQ - 无法在不更改多表分组的情况下将列添加到结果中

2 个答案: