按子句分组与Distinct() [英] Group by clause vs. Distinct()

问题描述

我的ASP.NET自定义列表控件从数据库View获取其值.检索数据的方法最终返回类型为List<Triplet>的对象作为控件的数据源.

My ASP.NET custom list control gets its values from a database View. The method which retrieves data finally returns an object of type List<Triplet> as a DataSource for the control.

我找到了该方法的三种可能的实现，它们似乎都可以正常工作并给出相同的结果.现在我不确定应该选择其中的哪个.

I figured out three possible implementations for that method which all seem to work fine and give the same results. Now I'm not sure which of them should be preferred.

重点是，我需要按字母顺序从查询中获得唯一的字符串，并且数据库中有很多个重复项.因此，我可以全部获取它们，然后执行Distinct()以获取唯一值...

The point is, that I need unique strings from the query in alphabetical order, and there are many duplicates in the db. So I could fetch them all and then perfom a Distinct() to get unique values...

public override object GetData()
{
    return 
    (
        from name in
        (
            from job in DBConnection.NewDataContext.vJobs
            where job.name != null 
            select job.name

        ).Distinct().OrderBy(s => s) 

        select new Triplet(name, name, ListType)

    ).ToList();
 }

...或者我可以使用group by子句并仅选择键:

...or I could use a group by clause and only select the keys:

public override object GetData()
{
    return 
    (
        from job in DBConnection.NewDataContext.vJobs 
        where job.name != null
        group job by job.name into names 
        orderby names.Key 
        select new Triplet(names.Key, names.Key, ListType)

    ).ToList();
}

我还想出了以下内容，它为Triplets使用了特殊的EqualityComparer.实际上，这是我的第一种方法，但是我并不真的喜欢它:

I also came up with the following, which uses a special EqualityComparer for the Triplets. Actually it was my first approach, but I didn't really like it:

public override object GetData()
{
    return
    (
        from job in DBConnection.NewDataContext.vJobs 
        where job.name != null
        select new Triplet(job.name, job.name, ListType)

    ).ToList().Distinct(new TripletComparer()).OrderBy(t => (string)t.First).ToList();
}

我认为按解决方案进行的工作大部分工作都留给了数据库(MS SQL Server)，这可能是优点还是缺点..我真的不知道.也许Distict()解决方案不得不将过多不必要的数据从db推送到我的方法中?

I think the goup-by-solution leaves most of the work to the database (MS SQL Server), which might be an advantage or disadvantage.. I don't really know. Maybe the Distict()-solutution suffers from having to push too much unneccessary data from the db to my method?

有哪些想法应该实施?似乎因为树木过多我看不到森林...

Any ideas which one should be implemented? It seems I just can't see the forest because of too many trees...

推荐答案

直到需要关注性能为止(即

Until there is a need to concern yourself with performance (that is, do not micro-optimize) then you should probably opt for the most readable solution which is clearly to call Distinct as that conveys your intent very clearly.

如果您真正关心性能，那么我建议您使用 profiler .

If you are truly concerned about the performance, then I suggest you perform some concrete benchmarks using a profiler.

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