[英]LINQ - Left Join, Group By, and Count
假設我有這個SQL:
SELECT p.ParentId, COUNT(c.ChildId)
FROM ParentTable p
LEFT OUTER JOIN ChildTable c ON p.ParentId = c.ChildParentId
GROUP BY p.ParentId
如何將其轉換為LINQ to SQL? 我被困在COUNT(c.ChildId),生成的SQL似乎總是輸出COUNT(*)。 這是我到目前為止所得到的:
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() }
謝謝!
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) }
考慮使用子查詢:
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 } ;
如果查詢類型通過關聯連接,則簡化為:
from p in context.ParentTable
let cCount = p.Children.Count()
select new { ParentId = p.Key, Count = cCount } ;
最后答復:
如果您所做的只是Count(),則根本不需要左連接 。 請注意, join...into
實際上被轉換為GroupJoin
,它返回分組,如new{parent,IEnumerable<child>}
因此您只需要在組上調用Count()
:
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() }
在擴展方法語法中, join into
等同於GroupJoin
(而沒有into
的join
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) })
雖然LINQ語法背后的想法是模擬SQL語法,但您不應該總是考慮直接將SQL代碼轉換為LINQ。 在這種特殊情況下,我們不需要進行分組,因為join into是一個組連接本身。
這是我的解決方案:
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() }
與此處的大多數投票解決方案不同,我們在Count中不需要j1 , j2和null檢查(t => t.ChildId!= null)
聲明:本站的技術帖子網頁,遵循CC BY-SA 4.0協議,如果您需要轉載,請注明本站網址或者原文地址。任何問題請咨詢:yoyou2525@163.com.