LINQ to SQL中内连接的语法是什么？

Question

我正在编写一个LINQ to SQL语句,我正在使用ONC#中的子句进行普通内连接的标准语法.

如何在LINQ to SQL中表示以下内容:

select DealerContact.*
from Dealer 
inner join DealerContact on Dealer.DealerID = DealerContact.DealerID

Answer 1

它类似于:

from t1 in db.Table1
join t2 in db.Table2 on t1.field equals t2.field
select new { t1.field2, t2.field3}

为表格提供合理的名称和字段会更好.:)

更新

我认为对于您的查询,这可能更合适:

var dealercontacts = from contact in DealerContact
                     join dealer in Dealer on contact.DealerId equals dealer.ID
                     select contact;

既然您正在寻找联系人,而不是经销商.

Answer 2

因为我更喜欢表达式链接语法,所以这是你如何做到这一点:

var dealerContracts = DealerContact.Join(Dealer, 
                                 contact => contact.DealerId,
                                 dealer => dealer.DealerId,
                                 (contact, dealer) => contact);

Answer 3

要扩展Clever Human 的表达链语法答案:

如果你想在两个表连接在一起的字段上做某些事情(比如过滤或选择) - 而不只是这两个表中的一个 - 你可以在Join方法的最后一个参数的lambda表达式中创建一个新对象合并这两个表格,例如:

var dealerInfo = DealerContact.Join(Dealer, 
                              dc => dc.DealerId,
                              d => d.DealerId,
                              (dc, d) => new { DealerContact = dc, Dealer = d })
                          .Where(dc_d => dc_d.Dealer.FirstName == "Glenn" 
                              && dc_d.DealerContact.City == "Chicago")
                          .Select(dc_d => new {
                              dc_d.Dealer.DealerID,
                              dc_d.Dealer.FirstName,
                              dc_d.Dealer.LastName,
                              dc_d.DealerContact.City,
                              dc_d.DealerContact.State });

有趣的部分是该示例的第4行中的lambda表达式:

(dc, d) => new { DealerContact = dc, Dealer = d }

...我们构造一个新的匿名类型对象,它具有DealerContact和Dealer记录的属性以及它们的所有字段.

然后,我们可以在筛选和选择结果时使用这些记录中的字段,如示例的其余部分所示,该示例使用dc_d我们构建的匿名对象的名称,该对象同时具有DealerContact和Dealer记录作为其属性.

Answer 4

var results = from c in db.Companies
              join cn in db.Countries on c.CountryID equals cn.ID
              join ct in db.Cities on c.CityID equals ct.ID
              join sect in db.Sectors on c.SectorID equals sect.ID
              where (c.CountryID == cn.ID) && (c.CityID == ct.ID) && (c.SectorID == company.SectorID) && (company.SectorID == sect.ID)
              select new { country = cn.Name, city = ct.Name, c.ID, c.Name, c.Address1, c.Address2, c.Address3, c.CountryID, c.CityID, c.Region, c.PostCode, c.Telephone, c.Website, c.SectorID, Status = (ContactStatus)c.StatusID, sector = sect.Name };


return results.ToList();

Answer 5

使用Linq Join运算符:

var q =  from d in Dealer
         join dc in DealerConact on d.DealerID equals dc.DealerID
         select dc;

Answer 6

您创建了一个外键,LINQ-to-SQL为您创建了导航属性.Dealer然后每个都有一个集合DealerContacts,您可以选择,过滤和操作.

from contact in dealer.DealerContacts select contact

要么

context.Dealers.Select(d => d.DealerContacts)

如果您没有使用导航属性,那么您将错过LINQ-to-SQL的主要优势之一 - 映射对象图的部分.

Answer 7

基本上LINQ join运算符对SQL没有任何好处.即以下查询

var r = from dealer in db.Dealers
   from contact in db.DealerContact
   where dealer.DealerID == contact.DealerID
   select dealerContact;

将导致SQL中的INNER JOIN

join对IEnumerable <>非常有用,因为它更有效:

from contact in db.DealerContact  

每个经销商都会重新执行该条款 但对于IQueryable <>则不是这样.还加入不够灵活.

Answer 8

实际上,通常最好不要加入linq.当有导航属性时,编写linq语句的一种非常简洁的方法是:

from dealer in db.Dealers
from contact in dealer.DealerContacts
select new { whatever you need from dealer or contact }

它转换为where子句:

SELECT <columns>
FROM Dealer, DealerContact
WHERE Dealer.DealerID = DealerContact.DealerID

Answer 9

linq C# 内连接两个表

var result = from q1 in table1
             join q2 in table2
             on q1.Customer_Id equals q2.Customer_Id
             select new { q1.Name, q1.Mobile, q2.Purchase, q2.Dates }