实体框架组按最大日期和计数 [英] Entity Framework Group By with Max Date and count

问题描述

我有以下SQL

SELECT  Tag , COUNT(*) , MAX(CreatedDate)
FROM    dbo.tblTags
GROUP BY Tag

哪些输出如下：

+-----------------+------------------+-------------------------+
|       Tag       | (No column name) |    (No column name)     |
+-----------------+------------------+-------------------------+
| a great tag     |                1 | 2015-04-01 18:30:31.623 |
| not a test      |                1 | 2015-04-01 17:46:09.360 |
| test            |                5 | 2015-04-01 18:13:17.920 |
| test2           |                1 | 2013-03-07 16:53:54.217 |
+-----------------+------------------+-------------------------+

我是尝试使用EntityFramework复制该查询的输出。

I'm trying to replicate the output of that query using EntityFramework.

我有以下逻辑：

    var GroupedTags = Tags.GroupBy(c => c.Tag)
        .Select(g => new 
        { 
            name = g.Key, 
            count = g.Count(), 
            date = g.OrderByDescending(gt => gt.CreatedDate).FirstOrDefault().CreatedDate 
        })
        .OrderBy(c => c.name);

与原始SQL查询相比，执行时间相当长。关于如何优化我的方法的任何建议？

But takes horribly long to execute compared to the raw SQL query. Any suggestions on how to optimise my approach? It somehow feels wrong.

推荐答案

如果您想要一个最大值，请使用 Max（） Linq方法：

If you want a max, use the Max() Linq method:

var GroupedTags = Tags.GroupBy(c => c.Tag)
    .Select(g => new 
    { 
        name = g.Key, 
        count = g.Count(), 
        date = g.Max(x => x.CreatedDate)
    })
    .OrderBy(c => c.name);

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