LINQ - 左连接,分组依据和计数

假设我有这个SQL:

SELECT p.ParentId, COUNT(c.ChildId)
FROM ParentTable p
  LEFT OUTER JOIN ChildTable c ON p.ParentId = c.ChildParentId
GROUP BY p.ParentId

如何将其转换为LINQ to SQL？我被困在COUNT(c.ChildId),生成的SQL似乎总是输出COUNT(*).这是我到目前为止所得到的:

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() }

谢谢!

1> Mehrdad Afsh..：

---

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) }

---

这就是SQL的工作原理.COUNT(fieldname)将计算该字段中非空的行.也许我没有得到您的问题,请澄清是否是这种情况.

2> Amy B..：

---

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 } ;

---

如果查询类型通过关联连接,则简化为:

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

3> Eren Ersönme..：

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最后答复:

你不应该需要的左连接在所有如果你正在做的是伯爵().请注意,join...into实际上它被转换为GroupJoin返回分组,new{parent,IEnumerable}因此您只需要调用Count()该组:

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join into等效于GroupJoin(而join没有一个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() }
    );

4> 小智..：

---

 (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) })

5> Mosh..：

---

虽然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.在这种特殊情况下,我们不需要进行分组,因为join into是一个组连接本身.

这是我的解决方案:

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() }

与此处的大多数投票解决方案不同,我们在Count中不需要j1,j2和null检查(t => t.ChildId!= null)