GROUP BY 및 COUNT (DISTINCT)를 사용하는 LINQ to SQL

GROUP BY 및 COUNT (DISTINCT)를 사용하는 LINQ to SQL

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다음 SQL 쿼리를 수행해야합니다.

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

LINQ to SQL 쿼리

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

다음 SQL 쿼리에 매핑됩니다.

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

여태까지는 그런대로 잘됐다. 그러나 여기에 작성한 첫 번째 쿼리에 매핑되는 LINQ to SQL 쿼리를 찾을 수 없기 때문에 결과를 그룹화하려고 할 때 문제가 발생합니다 ( 이 프로세스를 훨씬 쉽게 만든 LINQPad 에 감사드립니다 ). 다음은 원하는 결과를 제공하는 유일한 방법입니다.

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

차례로 다음과 같은 추악하고 최적화되지 않은 모든 SQL 쿼리를 생성합니다.

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

어떤 도움을 주시면 감사하겠습니다.

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거기에 대한 직접적인 지원은하지 않습니다 COUNT(DISTINCT {x})),하지만 당신은에서를 시뮬레이션 할 수있는 IGrouping<,>(즉, 무엇을 group by반환); C # 만 "실행"하는 것 같아서 VB로 번역해야합니다.

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

다음은 Northwind의 예입니다.

    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);
        }
    }

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

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);
        }

With definition:

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

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The Northwind example cited by Marc Gravell can be rewritten with the City column selected directly by the group statement:

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()
};

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Linq to sql has no support for Count(Distinct ...). You therefore have to map a .NET method in code onto a Sql server function (thus Count(distinct.. )) and use that.

btw, it doesn't help if you post pseudo code copied from a toolkit in a format that's neither VB.NET nor C#.

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This is how you do a distinct count query. Note that you have to filter out the nulls.

var useranswercount = (from a in tpoll_answer
where user_nbr != null && answer_nbr != null
select user_nbr).Distinct().Count();

If you combine this with into your current grouping code, I think you'll have your solution.

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simple and clean example of how group by works in LINQ

http://www.a2zmenu.com/LINQ/LINQ-to-SQL-Group-By-Operator.aspx

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I wouldn't bother doing it in Linq2SQL. Create a stored Procedure for the query you want and understand and then create the object to the stored procedure in the framework or just connect direct to it.

참고URL : https://stackoverflow.com/questions/448203/linq-to-sql-using-group-by-and-countdistinct