LINQ：在日期时间字段内按月份和年份分组

Question

我有一个带有日期时间字段的表。我想检索按月份/年份组合分组的结果集以及该月/年内出现的记录数。这怎么可以在LINQ中完成？

最近我已经能够找出在TSQL：

select substring(mo,charindex(mo,'/'),50) from (
select mo=convert(varchar(2),month(created)) + '/' + convert(varchar(4), year(created)) 
,qty=count(convert(varchar(2),month(created)) + '/' + convert(varchar(4), year(created))) 
from posts 
group by convert(varchar(2),month(created)) + '/' + convert(varchar(4), year(created)) 
) a 
order by substring(mo,charindex(mo,'/')+1,50) 

但我不会说，工程...

Answer 1

var grouped = from p in posts 
    group p by new { month = p.Create.Month,year= p.Create.Year } into d 
    select new { dt = string.Format("{0}/{1}",d.Key.month,d.Key.year), count = d.Count() }; 

下面是日期时间函数LINQ提供的列表。对于这个工作，你还需要了解multi-column grouping

勒令降

var grouped = (from p in posts 
    group p by new { month = p.Create.Month,year= p.Create.Year } into d 
    select new { dt = string.Format("{0}/{1}",d.Key.month,d.Key.year), count = d.Count()}).OrderByDescending (g => g.dt); 

Answer 2

This Site有应填写您需要的例子。

这是基本的语法：

from o in yg 
group o by o.OrderDate.Month into mg 
select new { Month = mg.Key, Orders = mg } 

Answer 3

你也可以这样来做

from o in yg 
group o by o.OrderDate.ToString("MMM yyyy") into mg 
select new { Month = mg.Key, Orders = mg } 

你的结果将是

{2014年1月，25} {Feb 2015，15} etc ...

Answer 4

这是为那些正在尝试完成但使用lambda表达式的人。

假设你已经有实体的集合，每个实体都有订购日期作为它的一个属性。

yourCollection 
// This will return the list with the most recent date first. 
.OrderByDescending(x => x.OrderDate) 
.GroupBy(x => new {x.OrderDate.Year, x.OrderDate.Month}) 
// Bonus: You can use this on a drop down 
.Select(x => new SelectListItem 
     { 
      Value = string.Format("{0}|{1}", x.Key.Year, x.Key.Month), 
      Text = string.Format("{0}/{1} (Count: {2})", x.Key.Year, x.Key.Month, x.Count()) 
     }) 
.ToList(); 

如果您不需要SelectListItem的集合，然后只用这一个替代选择：

.Select(x => string.Format("{0}/{1} (Count: {2})", x.Key.Year, x.Key.Month, x.Count())) 

Answer 5

这里是在分组的DateTime一个简单的解决方案。

List<leaveallview> lav = new List<leaveallview>(); 
lav = dbEntity.leaveallviews.Where(m =>m.created==alldate).ToList(); 
dynamic lav1 = lav.GroupBy(m=>m.created.Value.GetDateTimeFormats()).FirstOrDefault().ToList();