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
The LINQ to SQL query
from a in tpoll_answer
where a.poll_nbr = 16 select a.answer_nbr, a.user_nbr distinct
maps to the following SQL query:
select distinct answer_nbr, distinct user_nbr
from tpoll_answer
where poll_nbr = 16
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)
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
Any help will be more than appreciated.
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()
};
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);
}
}
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; }
}