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

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，一切都很好。但是，在尝试对结果进行 GROUP 时会出现问题，因为我无法找到映射到我在此处编写的第一个查询的 LINQ to SQL 查询（感谢LINQPad 使此过程变得更加容易）。以下是我发现的唯一一个给我想要的结果：

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

这又会产生以下丑陋且根本没有优化的 SQL 查询：

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

任何帮助将不胜感激。

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）；恐怕我只会“做”C#，所以你必须翻译成 VB...

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

这是一个罗斯文的例子：

    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，但它可以完成工作：

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

然而，结果是正确的——可以通过手动运行来验证：

        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);
        }

有定义：

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()
};

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（因此是 Count(distinct.. )）并使用它。

顺便说一句，如果您以既不是 VB.NET 也不是 C# 的格式发布从工具包复制的伪代码，这将无济于事。

Answer 4

这就是您执行不同计数查询的方式。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();

如果你把它与你当前的分组代码结合起来，我想你会有你的解决方案。

Answer 5

在 LINQ 中 group by 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

我不会费心在 Linq2SQL 中这样做。为您想要和理解的查询创建一个存储过程，然后在框架中为存储过程创建对象，或者直接连接到它。