C# 分组依据、计数和 Lambda 表达式
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Group by, Count and Lambda Expression
提问by N8K8
I am trying to translate the following query:
我正在尝试翻译以下查询：
SELECT STATE, COUNT(*)
FROM MYTABLE
GROUP BY STATE;
Into a lambda expression. I am using C# and EntityFramework, however it doesnt seem I can make it work. Here is what I have on my respository so far:
变成一个 lambda 表达式。我正在使用 C# 和 EntityFramework，但是我似乎无法让它工作。到目前为止，这是我的存储库中的内容：
public IEnumerable<object> PorcentajeState(Guid id)
{
return _context.Sates.Where(a => a.Id == id)
.GroupBy(a => a.State)
.Select(n => new { n.StateId , n.Count() });
}
Of course it doesnt compile and I am lost after googling for 2 hours . Could you please help me?
当然它不会编译，我在谷歌搜索 2 小时后迷路了。请你帮助我好吗？
thanks in advance
提前致谢
采纳答案by p.s.w.g
There are two issues here:
这里有两个问题：
- The result of
GroupBywill will be an enumerable of typeIEnumerable<IGrouping<TKey, TSource>>. TheIGroupinginterface only has one property you can access,Keywhich is the key you specified in theGroupByexpression, and implementsIEnumerable<T>so you can do other Linq operations on the result. - You need to specify a property name for the anonymous type if it cannot be inferred from a property or field expression. In this case, you're calling
Counton theIGrouping, so you need to specify a name for that property.
GroupBywill的结果将是一个可枚举类型IEnumerable<IGrouping<TKey, TSource>>。该IGrouping接口只有一个您可以访问的属性，Key即您在GroupBy表达式中指定的键，并实现了IEnumerable<T>您可以对结果执行其他 Linq 操作。- 如果无法从属性或字段表达式推断出匿名类型，则需要为匿名类型指定属性名称。在这种情况下，您调用
Count的是IGrouping，因此您需要为该属性指定一个名称。
Try this:
尝试这个：
public IEnumerable<object> PorcentajeState(Guid id)
{
return _context.Sates.Where(a => a.Id == id)
.GroupBy(a => a.StateId)
.Select(g => new { g.Key, Count = g.Count() });
}
The equivalent in query syntax would be
查询语法中的等效项是
public IEnumerable<object> PorcentajeState(Guid id)
{
return from a in _context.Sates
where a.Id == id
group a by a.StateId into g
select new { a.Key, Count = g.Count() };
}
In either case, if you want the first property to be named StateIdinstead of Key, just change that to
在任何一种情况下，如果您希望第一个属性被命名StateId而不是Key，只需将其更改为
new { StateId = g.Key, Count = g.Count() }
回答by Ahsan Qureshi
This one is good
这个不错
public IEnumerable<object> PorcentajeState(Guid id)
{
return _context.Sates.Where(a => a.Id == id)
.GroupBy(a => a.StateId)
.Select(g => new { g.Key, Count = g.Count() });
}
But try this.
但是试试这个。
public IEnumerable<object> PorcentajeState(Guid id)
{
return _context.Sates.Where(a => a.Id == id)
.GroupBy(a => a.StateId)
.Select(g => new { g.Key.StateId, Count = g.Count() });
}
