LINQ to SQL中使用GROUP BY和COUNT（DISTINCT） [英] LINQ to SQL using GROUP BY and COUNT(DISTINCT)

问题描述

我必须执行下面的SQL查询：

I have to perform the following SQL query:

select answer_nbr, count(distinct user_nbr)
from tpoll_answer
where poll_nbr = 16
group by answer_nbr

在LINQ to SQL查询

The LINQ to SQL query

from a in tpoll_answer 
where a.poll_nbr = 16 select a.answer_nbr, a.user_nbr distinct 

映射到下面的SQL查询：

maps to the following SQL query:

select distinct answer_nbr, distinct user_nbr
from tpoll_answer
where poll_nbr = 16

到目前为止，一切都很好。然而，问题试图GROUP时的结果，因为我不是能够找到一个LINQ to SQL查询映射到我写到这里的第一个查询提升（谢谢你的 LINQPad 使这一过程容易得多）。下面是我发现的唯一一个给我想要的结果：

So far, so good. However the problem raises when trying to GROUP the results, as I'm not being able to find a LINQ to SQL query that maps to the first query I wrote here (thank you LINQPad for making this process a lot easier). The following is the only one that I've found that gives me the desired result:

from answer in tpoll_answer where answer.poll_nbr = 16 _
group by a_id = answer.answer_nbr into votes = count(answer.user_nbr)

这反过来产生follwing丑陋和所有SQL查询非优化：

Which in turns produces the follwing ugly and non-optimized at all SQL query:

SELECT [t1].[answer_nbr] AS [a_id], (
    SELECT COUNT(*)
    FROM (
        SELECT CONVERT(Bit,[t2].[user_nbr]) AS [value], [t2].[answer_nbr], [t2].[poll_nbr]
        FROM [TPOLL_ANSWER] AS [t2]
        ) AS [t3]
    WHERE ([t3].[value] = 1) AND ([t1].[answer_nbr] = [t3].[answer_nbr]) AND ([t3].[poll_nbr] = @p0)
    ) AS [votes]
FROM (
    SELECT [t0].[answer_nbr]
    FROM [TPOLL_ANSWER] AS [t0]
    WHERE [t0].[poll_nbr] = @p0
    GROUP BY [t0].[answer_nbr]
    ) AS [t1]
-- @p0: Input Int (Size = 0; Prec = 0; Scale = 0) [16]
-- Context: SqlProvider(Sql2008) Model: AttributedMetaModel Build: 3.5.30729.1

任何帮助将超过AP preciated。

Any help will be more than appreciated.

推荐答案

有不适合 COUNT直接支持（DISTINCT {X}）），但你可以模拟它从 IGrouping＆LT;，＆GT; （即什么按收益组）;恐怕我只做C＃，所以你必须要转换成VB ...

There isn't direct support for COUNT(DISTINCT {x})), but you can simulate it from an IGrouping<,> (i.e. what group by returns); I'm afraid I only "do" C#, so you'll have to translate to VB...

 select new
 {
     Foo= grp.Key,
     Bar= grp.Select(x => x.SomeField).Distinct().Count()
 };

下面是一个罗斯文例如：

Here's a Northwind example:

    using(var ctx = new DataClasses1DataContext())
    {
        ctx.Log = Console.Out; // log TSQL to console
        var qry = from cust in ctx.Customers
                  where cust.CustomerID != ""
                  group cust by cust.Country
                  into grp
                  select new
                  {
                      Country = grp.Key,
                      Count = grp.Select(x => x.City).Distinct().Count()
                  };

        foreach(var row in qry.OrderBy(x=>x.Country))
        {
            Console.WriteLine("{0}: {1}", row.Country, row.Count);
        }
    }

该TSQL不太我们想要的东西，但它的工作：

The TSQL isn't quite what we'd like, but it does the job:

SELECT [t1].[Country], (
    SELECT COUNT(*)
    FROM (
        SELECT DISTINCT [t2].[City]
        FROM [dbo].[Customers] AS [t2]
        WHERE ((([t1].[Country] IS NULL) AND ([t2].[Country] IS NULL)) OR (([t1]
.[Country] IS NOT NULL) AND ([t2].[Country] IS NOT NULL) AND ([t1].[Country] = [
t2].[Country]))) AND ([t2].[CustomerID] <> @p0)
        ) AS [t3]
    ) AS [Count]
FROM (
    SELECT [t0].[Country]
    FROM [dbo].[Customers] AS [t0]
    WHERE [t0].[CustomerID] <> @p0
    GROUP BY [t0].[Country]
    ) AS [t1]
-- @p0: Input NVarChar (Size = 0; Prec = 0; Scale = 0) []
-- Context: SqlProvider(Sql2008) Model: AttributedMetaModel Build: 3.5.30729.1

结果，却是正确─verifyable通过手动运行它：

The results, however, are correct- verifyable by running it manually:

        const string sql = @"
SELECT c.Country, COUNT(DISTINCT c.City) AS [Count]
FROM Customers c
WHERE c.CustomerID != ''
GROUP BY c.Country
ORDER BY c.Country";
        var qry2 = ctx.ExecuteQuery<QueryResult>(sql);
        foreach(var row in qry2)
        {
            Console.WriteLine("{0}: {1}", row.Country, row.Count);
        }

通过定义：

class QueryResult
{
    public string Country { get; set; }
    public int Count { get; set; }
}

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