Question

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

到目前为止，这么好。然而，当尝试对结果进行GROUP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使这个过程变得更容易）。以下是我发现的唯一能给我预期效果的结果：

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

在所有SQL查询中，这反过来会产生丑陋和非优化：

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

任何帮助都将得到更多的赞赏。

Answer 1

没有对COUNT(DISTINCT {x}))的直接支持，但您可以从IGrouping<,>模拟它（即group by返回的内容）;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并不是我们想要的，但它确实起到了作用：

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

然而，通过手动运行结果是正确的：

        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);
        }

定义：

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

Answer 2

Marc Gravell引用的Northwind示例可以使用由群组声明直接选择的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()
};

Answer 3

Linq to sql不支持Count（Distinct ...）。因此，您必须将代码中的.NET方法映射到Sql服务器函数（因此为Count（distinct ..））并使用它。

顺便说一下，如果你发布从工具箱中复制的伪代码，它既不是VB.NET也不是C＃的格式，它无济于事。

Answer 4

这是您执行非常计数查询的方式。请注意，您必须过滤掉空值。

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

如果将此与您当前的分组代码结合使用，我认为您将获得解决方案。

Answer 5

在LINQ中如何工作的简单而干净的示例

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

Answer 6

我不想在Linq2SQL中做这件事。为您想要和理解的查询创建存储过程，然后在框架中创建存储过程的对象，或者直接连接到它。

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

6 个答案: