C# 在 LINQ 中选择计数
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Selecting count in LINQ
提问by SeToY
I got a SQL Server table with columns ResolvedDateand ResolvedBy.
我得到了一个包含列ResolvedDate和ResolvedBy.
Now I want to select those two columns and count their results, which I thought I'd implement like that:
现在我想选择这两列并计算它们的结果，我想我会这样实现：
_dataContext.Activities
.Where(a => a.IsResolved && a.ResolvedBy == userId)
.Select(a => new { a.ResolvedDate, ***COUNT*** });
As you can see, the Count-Part is missing.
如您所见，缺少 Count-Part。
I want to get every activity, that has been done in this ResolvedDate and count it.
我想获取在此 ResolvedDate 中完成的所有活动并对其进行计数。
How can I implement that?
我该如何实施？
采纳答案by Ralph Shillington
from a in dataContext.Activities
where a.IsResolved && a.ResolvedBy == userId
group a by a.ResolvedDate into g
select new {ResolvedOn=g.Key, NumberResolved= g.Count()}
回答by SLaks
var dates = dataContext.Activities
.Where(a => a.IsResolved && a.ResolvedBy == userId)
.Select(a => a.ResolvedDate)
.ToList();
var count = dates.Count;
If you only want to get the count, you can make it much faster by selecting the count alone:
如果你只想得到计数，你可以通过单独选择计数来使它更快：
var count = dataContext.Activities
.Count(a => a.IsResolved && a.ResolvedBy == userId);
回答by Reed Copsey
If you're trying to count each item by date, you'd need to use GroupBy:
如果您尝试按日期计算每个项目，则需要使用GroupBy：
var countsByDate = _dateContext.Activities
.Where(a => a.IsResolved && a.ResolvedBy == userId)
.GroupBy(a => a.ResolvedDate)
.Select(g => new {ResolvedDate = g.Key, Count = g.Count() });
回答by MarcinJuraszek
You have to group your data by ResolvedDate to get number of activities resolved every day.
您必须按 ResolvedDate 对数据进行分组，以获得每天解决的活动数量。
var dates = from a in dataContext.Activities
where a.IsResolved && a.ResolvedBy == userId
group a by a.ResolvedDate into g
select new { Date = g.Key, Count = g.Count() }
To group just by day (without hour, minutes, etc.) you can change the group statement:
要按天分组（没有小时、分钟等），您可以更改组语句：
var dates = from a in dataContext.Activities
where a.IsResolved && a.ResolvedBy == userId
group a by a.ResolvedDate.Date into g
select new { Date = g.Key, Count = g.Count() }
