[英]Select Latest Item for Each Type
这是基于Todilo先前的问题 。
以下接受的答案非常有用,除了我需要返回所有类型为null的所有记录以及每种类型的最新记录之外:
var query = Posts.GroupBy(p => p.Type)
.Select(g => g.OrderByDescending(p => p.Date)
.FirstOrDefault()
)
场景如下:
+----+--------------------------+-------+------------+
| id | content | type | date |
+----+--------------------------+-------+------------+
| 0 | Some text | TypeA | 2013-04-01 |
| 1 | Some older text | TypeA | 2012-03-01 |
| 2 | Some even older texttext | TypeA | 2011-01-01 |
| 3 | Sample | | 2013-02-24 |
| 4 | A dog | TypeB | 2013-04-01 |
| 5 | And older dog | TypeB | 2012-03-01 |
| 6 | An even older dog | TypeB | 2011-01-01 |
| 7 | Another sample | | 2014-03-06 |
| 8 | Test | | 2015-11-08 |
+----+--------------------------+-------+------------+
结果应该是
Some text | TypeA
Sample |
A dog | TypeB
Another sample |
Test |
那个怎么样:
var query = Posts
.GroupBy(p => p.Type)
.Select(g => g.OrderByDescending(p => p.Date).FirstOrDefault()).ToList()
var lostNullItems = Posts.Where(p => p.Type == null && !query.Contains(p));
var newQuery = query.Union(lostNullItems);
如果您不需要项目的顺序,可以使用:
var query = Posts
.GroupBy(p => p.Type)
.SelectMany(g =>
{
var result = g.OrderByDescending(p => p.Date);
return g.Key == null ? result ? Enumerable.Repeat(result.First(), 1);
});
此代码未经测试。
请尝试以下代码。 由于分组的顺序不同
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data;
namespace ConsoleApplication1
{
class Program
{
static void Main(string[] args)
{
DataTable dt = new DataTable();
dt.Columns.Add("id", typeof(int));
dt.Columns.Add("content", typeof(string));
dt.Columns.Add("type", typeof(string));
dt.Columns["type"].AllowDBNull = true;
dt.Columns.Add("date", typeof(DateTime));
dt.Rows.Add(new object[] { 0, "Some text", "TypeA", DateTime.Parse("2013-04-01")});
dt.Rows.Add(new object[] { 1, "Some older text", "TypeA", DateTime.Parse("2012-03-01")});
dt.Rows.Add(new object[] { 2, "Some older texttext", "TypeA", DateTime.Parse("2011-01-01")});
dt.Rows.Add(new object[] { 3, "Sample", null, DateTime.Parse("2013-02-24")});
dt.Rows.Add(new object[] { 3, "A dog", "TypeB", DateTime.Parse("2013-04-01")});
dt.Rows.Add(new object[] { 4, "And older dog", "TypeB", DateTime.Parse("2012-03-01")});
dt.Rows.Add(new object[] { 5, "An even older dog", "TypeB", DateTime.Parse("2011-01-01")});
dt.Rows.Add(new object[] { 4, "Another sample", null, DateTime.Parse("2014-03-06")});
dt.Rows.Add(new object[] { 5, "Test", null, DateTime.Parse("2015-11-08")});
var results = dt.AsEnumerable()
.GroupBy(x => x.Field<string>("type"))
.Select(x => x.Key == null ? x.ToList() : x.Select(y => new {date = y.Field<DateTime>("date"), row = y}).OrderByDescending(z => z.date).Select(a => a.row).Take(1))
.SelectMany(b => b).Select(c => new {
content = c.Field<string>("content"),
type = c.Field<string>("type")
}).ToList();
}
}
}
我认为LINQ union
是唯一的方法,这主要是因为您如何按日期对输出进行排序。
第一个查询应如下所示:
var nullTypes = from p in Posts
where p.Type == null
select p;
主要查询应仅过滤出空值:
var query = Posts.Where(p => p.Type != null).GroupBy(p => p.Type)
.Select(g => g.OrderByDescending(p => p.Date)
.FirstOrDefault()
)
联合到您的主要查询:
var unionQuery = query.Union(nullTypes).OrderBy(p => p.Date);
输出将符合您的期望,只是前两行将按顺序反转:
Sample | Some text | TypeA A dog | TypeB Another sample | Test |
这是因为包含“示例”的订单项的日期早于“某些文本”的日期。
您不需要做任何SelectMany
原始的Select
将SelectMany
单个项目,因此可以算出一个适合联合的IEnumerable。
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