Linq-左联接、分组依据和计数

from p in context.ParentTable
join c in context.ChildTable on p.ParentId equals c.ChildParentId into j1
from j2 in j1.DefaultIfEmpty()
group j2 by p.ParentId into grouped
select new { ParentId = grouped.Key, Count = grouped.Count(t=>t.ChildId != null) }

考虑使用子查询:

from p in context.ParentTable 
let cCount =
(
  from c in context.ChildTable
  where p.ParentId == c.ChildParentId
  select c
).Count()
select new { ParentId = p.Key, Count = cCount } ;

如果查询类型是通过关联连接的,这将简化为:

from p in context.ParentTable 
let cCount = p.Children.Count()
select new { ParentId = p.Key, Count = cCount } ;

迟到的回答:

你 不需要左边的加入 如果你所做的一切都是count()。注意 join...into 实际上被翻译成 GroupJoin 它返回如下分组 new{parent,IEnumerable<child>} 所以你只需要打电话 Count() 小组:

from p in context.ParentTable
join c in context.ChildTable on p.ParentId equals c.ChildParentId into g
select new { ParentId = p.Id, Count = g.Count() }

在扩展方法语法A中 join into 等于 组联接 (而A) join 没有 into 是 Join ):

context.ParentTable
    .GroupJoin(
                   inner: context.ChildTable
        outerKeySelector: parent => parent.ParentId,
        innerKeySelector: child => child.ParentId,
          resultSelector: (parent, children) => new { parent.Id, Count = children.Count() }
    );

 (from p in context.ParentTable     
  join c in context.ChildTable 
    on p.ParentId equals c.ChildParentId into j1 
  from j2 in j1.DefaultIfEmpty() 
     select new { 
          ParentId = p.ParentId,
         ChildId = j2==null? 0 : 1 
      })
   .GroupBy(o=>o.ParentId) 
   .Select(o=>new { ParentId = o.key, Count = o.Sum(p=>p.ChildId) })

尽管Linq语法背后的想法是模拟SQL语法,但您不应该总是考虑直接将SQL代码转换为Linq。在这种情况下,我们不需要这样做 组入 自从 加入 是一个组联接本身。

我的解决方案是:

from p in context.ParentTable
join c in context.ChildTable on p.ParentId equals c.ChildParentId into joined
select new { ParentId = p.ParentId, Count = joined.Count() }

与这里多数人投票的解决方案不同,我们不需要 J1 , J2 和空签入 计数(t=>t.childID!=空)