使用GROUP BY和COUNT(DISTINCT)的LINQ to SQL

我必须执行以下SQL查询:

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

LINQ to SQL查询

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

映射到以下SQL查询:

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

到现在为止还挺好.但是在尝试GROUP结果时问题就出现了,因为我无法找到映射到我在这里写的第一个查询的LINQ to SQL查询(感谢LINQPad让这个过程变得更容易).以下是我发现的唯一一个给我预期结果的:

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

这反过来会产生所有SQL查询中的丑陋和非优化:

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

任何帮助将不仅仅是赞赏.

没有直接的支持COUNT(DISTINCT {x})),但你可以从一个模拟它IGrouping<,>(即什么group by返回); 我担心我只会"做"C#,所以你必须翻译成VB ...

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

这是一个Northwind示例:

    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);
        }
    }

TSQL不是我们想要的,但是它完成了这项工作:

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

但是,通过手动运行结果可以正确验证结果:

        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(sql);
        foreach(var row in qry2)
        {
            Console.WriteLine("{0}: {1}", row.Country, row.Count);
        }

定义:

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

Marc Gravell引用的Northwind示例可以使用group语句直接选择的City列重写:

from cust in ctx.Customers
where cust.CustomerID != ""
group cust.City /*here*/ by cust.Country
into grp
select new
{
        Country = grp.Key,
        Count = grp.Distinct().Count()
};