linq sql-总数

Question

I have some linq that returns the correct data.

var numEmails = (from row in EmailBatchProposal  
where row.EmailBatchId == emailBatchId
select row.EmailBatchProposalId).Count();

However, if I understand linq correctly, this does not perform optimally. It grabs all the data and then walks through the list and counts the rows. What I'd really like is for linq (in the background) to use like:

Select count(*) from ...

I trust the performance reasons are obvious.

Does anyone know the proper way to do this?

Answer 1

People generally learn best by practicing. I would suggest you get a copy of LinqPad (free), enter in your Linq query, and see what SQL it generates. Then you can modify the Linq query until you get exactly what you want.

Answer 2

实际上，如果linq查询与实现IQueryable并支持转换为基础SQL变体的集合一起使用，则是正确转换示例中的Count函数的相当基本的功能。

Answer 3

You can just use the argument of .Count() .

int numEmails = EmailBatchProposal.Count(x => x.EmailBatchId == emailBatchId)

As noted below, this doesn't actually resolve to any different SQL, but I think it's at least a cleaner looking alternative.

Answer 4

Actually, the LINQ-to-SQL is smart enough to know that it should do a count... For example, if I have the following query:

var v = (from u in TblUsers
        select u).Count();

the SQL that actually executes when I run this is:

SELECT COUNT(*) AS [value]
FROM [tblUsers] AS [t0]

Amazing, huh? Another poster made the really good suggestion of getting LinqPad - it is a WONDERFUL tool. It will show you the exact SQL that gets executed. You can always confirm with SQL profiler too.

Answer 5

Check the Log of the SQL used in the query.

        using (var dbc = new siteDataContext())
        {
            dbc.Log = Console.Out;
            var bd = (from b in dbc.birthdays
                     select b).Count();
            Console.WriteLine("{0}", bd);
        }

gives the query:

SELECT COUNT(*) AS [value]
FROM [dbo].[birthdays] AS [t0]
 -- Context: SqlProvider(Sql2008) Model: AttributedMetaModel Build: 3.5.30729.1