如何在c# LINQ中执行左外连接到对象而不使用join-on-equal -into子句?有办法用where子句来实现吗?
正确的问题:
内连接很简单,我有一个这样的解决方案
List<JoinPair> innerFinal = (from l in lefts from r in rights where l.Key == r.Key
select new JoinPair { LeftId = l.Id, RightId = r.Id})
但是对于左外连接,我需要一个解决方案。我的是这样的,但它不工作
List< JoinPair> leftFinal = (from l in lefts from r in rights
select new JoinPair {
LeftId = l.Id,
RightId = ((l.Key==r.Key) ? r.Id : 0
})
其中JoinPair是一个类:
public class JoinPair { long leftId; long rightId; }
根据我对类似问题的回答,在这里:
Linq到SQL左外连接使用Lambda语法和连接2列(复合连接键)
获得代码在这里,或克隆我的github回购,并发挥!
查询:
var petOwners =
from person in People
join pet in Pets
on new
{
person.Id,
person.Age,
}
equals new
{
pet.Id,
Age = pet.Age * 2, // owner is twice age of pet
}
into pets
from pet in pets.DefaultIfEmpty()
select new PetOwner
{
Person = person,
Pet = pet,
};
λ:
var petOwners = People.GroupJoin(
Pets,
person => new { person.Id, person.Age },
pet => new { pet.Id, Age = pet.Age * 2 },
(person, pet) => new
{
Person = person,
Pets = pet,
}).SelectMany(
pet => pet.Pets.DefaultIfEmpty(),
(people, pet) => new
{
people.Person,
Pet = pet,
});
这是一般形式(已经在其他答案中提供了)
var c =
from a in alpha
join b in beta on b.field1 equals a.field1 into b_temp
from b_value in b_temp.DefaultIfEmpty()
select new { Alpha = a, Beta = b_value };
然而,这里有一个解释,我希望能澄清这实际上是什么意思!
join b in beta on b.field1 equals a.field1 into b_temp
本质上创建了一个单独的结果集b_temp,其中有效地包含了右边条目的空“rows”('b'中的条目)。
然后是下一行:
from b_value in b_temp.DefaultIfEmpty()
..遍历该结果集,为右侧的'row'设置默认空值,并将右侧的行连接的结果设置为'b_value'的值(即,如果有匹配的记录,则为右侧的值,如果没有则为'null')。
现在,如果右边是一个单独的LINQ查询的结果,它将由匿名类型组成,它只能是'something'或'null'。如果它是一个可枚举的对象(例如,一个List -其中MyObjectB是一个有2个字段的类),那么它可以具体说明它的属性使用什么默认的“null”值:
var c =
from a in alpha
join b in beta on b.field1 equals a.field1 into b_temp
from b_value in b_temp.DefaultIfEmpty( new MyObjectB { Field1 = String.Empty, Field2 = (DateTime?) null })
select new { Alpha = a, Beta_field1 = b_value.Field1, Beta_field2 = b_value.Field2 };
这确保了'b'本身不是空的(但它的属性可以是空的,使用您指定的默认空值),这允许您检查b_value的属性,而不会得到b_value的空引用异常。注意,对于可空的DateTime, (DateTime?)类型,即。'nullable DateTime'必须在'DefaultIfEmpty'的规范中指定为null的'Type'(这也适用于'原生'不为空的类型,例如double, float)。
您可以通过简单地链接上述语法来执行多个左外连接。
概述:在这个代码片段中,我演示了如何按ID分组,其中Table1和Table2具有一对多关系。我集中在
Id, Field1和Field2。如果需要第三次查找表,并且需要左连接关系,那么子查询是有用的。
我展示了一个左连接分组和一个子查询linq。结果是等价的。
class MyView
{
public integer Id {get,set};
public String Field1 {get;set;}
public String Field2 {get;set;}
public String SubQueryName {get;set;}
}
IList<MyView> list = await (from ci in _dbContext.Table1
join cii in _dbContext.Table2
on ci.Id equals cii.Id
where ci.Field1 == criterion
group new
{
ci.Id
} by new { ci.Id, cii.Field1, ci.Field2}
into pg
select new MyView
{
Id = pg.Key.Id,
Field1 = pg.Key.Field1,
Field2 = pg.Key.Field2,
SubQueryName=
(from chv in _dbContext.Table3 where chv.Id==pg.Key.Id select chv.Field1).FirstOrDefault()
}).ToListAsync<MyView>();
Compared to using a Left Join and Group new
IList<MyView> list = await (from ci in _dbContext.Table1
join cii in _dbContext.Table2
on ci.Id equals cii.Id
join chv in _dbContext.Table3
on cii.Id equals chv.Id into lf_chv
from chv in lf_chv.DefaultIfEmpty()
where ci.Field1 == criterion
group new
{
ci.Id
} by new { ci.Id, cii.Field1, ci.Field2, chv.FieldValue}
into pg
select new MyView
{
Id = pg.Key.Id,
Field1 = pg.Key.Field1,
Field2 = pg.Key.Field2,
SubQueryName=pg.Key.FieldValue
}).ToListAsync<MyView>();
我想补充的是,如果您获得MoreLinq扩展,现在支持同质和异构左连接
http://morelinq.github.io/2.8/ref/api/html/Overload_MoreLinq_MoreEnumerable_LeftJoin.htm
例子:
//Pretend a ClientCompany object and an Employee object both have a ClientCompanyID key on them
return DataContext.ClientCompany
.LeftJoin(DataContext.Employees, //Table being joined
company => company.ClientCompanyID, //First key
employee => employee.ClientCompanyID, //Second Key
company => new {company, employee = (Employee)null}, //Result selector when there isn't a match
(company, employee) => new { company, employee }); //Result selector when there is a match
编辑:
回想起来,这可能会起作用,但它将IQueryable转换为IEnumerable,因为morelinq不会将查询转换为SQL。
您可以使用如下所述的GroupJoin: https://stackoverflow.com/a/24273804/4251433
这将确保它仍然是一个IQueryable,以防您以后需要对它进行进一步的逻辑操作。
使用lambda表达式
db.Categories
.GroupJoin(db.Products,
Category => Category.CategoryId,
Product => Product.CategoryId,
(x, y) => new { Category = x, Products = y })
.SelectMany(
xy => xy.Products.DefaultIfEmpty(),
(x, y) => new { Category = x.Category, Product = y })
.Select(s => new
{
CategoryName = s.Category.Name,
ProductName = s.Product.Name
});