LINQ to SQL中内连接的语法是什么？-Java 学习之路

399

我'm writing a LINQ to SQL statement, and I'm之后是C＃中带有 ON 子句的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

18 回答

var list = (from u in db.Users join c in db.Customers on u.CustomerId equals c.CustomerId where u.Username == username
   select new {u.UserId, u.CustomerId, u.ClientId, u.RoleId, u.Username, u.Email, u.Password, u.Salt, u.Hint1, u.Hint2, u.Hint3, u.Locked, u.Active,c.ProfilePic}).First();

编写所需的表名，并初始化select以获取字段的结果 .

回复于 2024-04-25T06:29:12+08:00

在linq C＃中内连接两个表

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 }

回复于 2024-04-25T06:29:12+08:00

一个最好的例子

表名： TBL_Emp 和 TBL_Dep

var result = from emp in TBL_Emp join dep in TBL_Dep on emp.id=dep.id
select new
{
 emp.Name;
 emp.Address
 dep.Department_Name
}


foreach(char item in result)
 { // to do}

回复于 2024-04-25T06:29:12+08:00

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 . :)

Update

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 .

回复于 2024-04-25T06:29:12+08:00

-1

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);

回复于 2024-04-25T06:29:12+08:00

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();

回复于 2024-04-25T06:29:12+08:00

2
要扩展Clever Human的表达链语法answer：

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，当我们过滤并选择结果时，我们可以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 作为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 .
回复于 2024-04-25T06:29:12+08:00

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;

回复于 2024-04-25T06:29:12+08:00

38
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_1771970都会重新执行子句 . 但是对于IQueryable <>，情况并非如此 . join 也不太灵活 .
回复于 2024-04-25T06:29:12+08:00
23
您创建了一个外键，LINQ-to-SQL为您创建了导航属性 . 然后每个 Dealer 都会有一个 DealerContacts 的集合，您可以选择，过滤和操作它们 .
```
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 .
回复于 2024-04-25T06:29:12+08:00

-6

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

回复于 2024-04-25T06:29:12+08:00

207

使用LINQ joins执行内部联接 .

var employeeInfo = from emp in db.Employees
                   join dept in db.Departments
                   on emp.Eid equals dept.Eid 
                   select new
                   {
                    emp.Ename,
                    dept.Dname,
                    emp.Elocation
                   };

回复于 2024-04-25T06:29:12+08:00

试试这个：

var data =(from t1 in dataContext.Table1 join 
                 t2 in dataContext.Table2 on 
                 t1.field equals t2.field 
                 orderby t1.Id select t1).ToList();

回复于 2024-04-25T06:29:12+08:00

OperationDataContext odDataContext = new OperationDataContext();    
        var studentInfo = from student in odDataContext.STUDENTs
                          join course in odDataContext.COURSEs
                          on student.course_id equals course.course_id
                          select new { student.student_name, student.student_city, course.course_name, course.course_desc };

学生和课程表具有主键和外键关系

回复于 2024-04-25T06:29:12+08:00

516

试试这个，

var dealer = from d in Dealer
             join dc in DealerContact on d.DealerID equals dc.DealerID
             select d;

回复于 2024-04-25T06:29:12+08:00

var q=(from pd in dataContext.tblProducts join od in dataContext.tblOrders on pd.ProductID equals od.ProductID orderby od.OrderID select new { od.OrderID,
 pd.ProductID,
 pd.Name,
 pd.UnitPrice,
 od.Quantity,
 od.Price,
 }).ToList();

回复于 2024-04-25T06:29:12+08:00

var Data= (from dealer in Dealer join dealercontact in DealerContact on dealer.ID equals dealercontact.DealerID
select new{
dealer.Id,
dealercontact.ContactName

}).ToList();

回复于 2024-04-25T06:29:12+08:00

var data=(from t in db.your tableName(t1) 
          join s in db.yourothertablename(t2) on t1.fieldname equals t2.feldname
          (where condtion)).tolist();

回复于 2024-04-25T06:29:12+08:00

LINQ to SQL中内连接的语法是什么？

18 回答

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