我有以下SQL,我正在尝试将其转换为LINQ:
SELECT f.value
FROM period as p
LEFT OUTER JOIN facts AS f ON p.id = f.periodid AND f.otherid = 17
WHERE p.companyid = 100
我已经看到了左外部联接(即into x from y in x.DefaultIfEmpty()
等)的典型实现,但是不确定如何引入其他联接条件(AND f.otherid = 17
)
编辑
为什么AND f.otherid = 17
条件部分是JOIN而不是WHERE子句中的?因为某些行可能不存在f
,但我仍然希望包含这些行。如果在WHERE子句中应用了该条件,则在JOIN之后-那么我没有得到想要的行为。
不幸的是:
from p in context.Periods
join f in context.Facts on p.id equals f.periodid into fg
from fgi in fg.DefaultIfEmpty()
where p.companyid == 100 && fgi.otherid == 17
select f.value
似乎与此等效:
SELECT f.value
FROM period as p
LEFT OUTER JOIN facts AS f ON p.id = f.periodid
WHERE p.companyid = 100 AND f.otherid = 17
这不是我想要的。
您需要在加入DefaultIfEmpty()
之前介绍加入条件。我只会使用扩展方法语法:
DefaultIfEmpty()
或者您可以使用子查询:
from p in context.Periods
join f in context.Facts on p.id equals f.periodid into fg
from fgi in fg.Where(f => f.otherid == 17).DefaultIfEmpty()
where p.companyid == 100
select f.value
这也可以,如果您有多个列连接,则也可以
from p in context.Periods
join f in context.Facts on p.id equals f.periodid into fg
from fgi in (from f in fg
where f.otherid == 17
select f).DefaultIfEmpty()
where p.companyid == 100
select f.value
我知道这是“ 有点晚了”,但以防万一有人需要使用LINQ方法语法(这是我最初找到这篇文章的原因)的情况,怎么做:
from p in context.Periods
join f in context.Facts
on new {
id = p.periodid,
p.otherid
} equals new {
f.id,
f.otherid
} into fg
from fgi in fg.DefaultIfEmpty()
where p.companyid == 100
select f.value
另一个有效的选择是将联接分布在多个LINQ子句之间,如下所示:
var results = context.Periods
.GroupJoin(
context.Facts,
period => period.id,
fk => fk.periodid,
(period, fact) => fact.Where(f => f.otherid == 17)
.Select(fact.Value)
.DefaultIfEmpty()
)
.Where(period.companyid==100)
.SelectMany(fact=>fact).ToList();
可以使用复合连接键编写。另外,如果需要从左右两侧选择属性,则LINQ可以写为
public static IEnumerable<Announcementboard> GetSiteContent(string pageName, DateTime date)
{
IEnumerable<Announcementboard> content = null;
IEnumerable<Announcementboard> addMoreContent = null;
try
{
content = from c in DB.Announcementboards
//Can be displayed beginning on this date
where c.Displayondate > date.AddDays(-1)
//Doesn't Expire or Expires at future date
&& (c.Displaythrudate == null || c.Displaythrudate > date)
//Content is NOT draft, and IS published
&& c.Isdraft == "N" && c.Publishedon != null
orderby c.Sortorder ascending, c.Heading ascending
select c;
//Get the content specific to page names
if (!string.IsNullOrEmpty(pageName))
{
addMoreContent = from c in content
join p in DB.Announceonpages on c.Announcementid equals p.Announcementid
join s in DB.Apppagenames on p.Apppagenameid equals s.Apppagenameid
where s.Apppageref.ToLower() == pageName.ToLower()
select c;
}
//CROSS-JOIN this content
content = content.Union(addMoreContent);
//Exclude dupes - effectively OUTER JOIN
content = content.Distinct();
return content;
}
catch (MyLovelyException ex)
{
throw ex;
}
}
在我看来,在尝试翻译SQL代码之前考虑对其进行一些重写是很有价值的。
就我个人而言,我会编写这样的查询作为联合(尽管我会完全避免使用null!):
var result = context.Periods
.Where(p => p.companyid == 100)
.GroupJoin(
context.Facts,
p => new {p.id, otherid = 17},
f => new {id = f.periodid, f.otherid},
(p, f) => new {p, f})
.SelectMany(
pf => pf.f.DefaultIfEmpty(),
(pf, f) => new MyJoinEntity
{
Id = pf.p.id,
Value = f.Value,
// and so on...
});
所以我想我同意@ MAbraham1的回答的精神(尽管他们的代码似乎与问题无关)。>>
但是,查询似乎已明确设计为产生包含重复行的单列结果-实际上是重复的null!很难不得出这种方法有缺陷的结论。