Question

我需要将以下SQL转换为LINQ：

Select Person.*, Governorate.OptionName
from Person 
Left Join Option Governorate on Governorate.OptionListName='Governorate' 
and Person.GovernorateId=Governorate.GovernorateId

我做了以下事情：

from person in db.persons
join governorate in db.Options 
on  new { Key1=person.GovernorateId,
         Key2=true} equals
    new { Key1=governorate.OptionValue,
         Key2= governorate.OptionListName equals "Governorate"}
select person, governorate

但这会产生两个错误：
1-对于：Key2= governorate.OptionListName equals "Governorate" 为“省”声明的equals和Anonymus类型的语法错误

2- For：select person, governorate 省不能在这个范围内宣布，因为它会赋予省不同的意义。

Answer 1

from person in db.persons
   join governorate in db.Options 
       on  new { Key1=person.GovernorateId, Key2="Governorate"} equals
new { Key1=governorate.OptionValue, Key2= governorate.OptionListName  }
into g
select new {
             person = person, 
             governorates = g
           }

它为您提供了一组对象，每个对象都有一个person和一个与governorate对应的集合。

如果您希望结果包含person和governorate对

from person in db.persons
   join governorate in db.Options 
       on  new { Key1=person.GovernorateId, Key2="Governorate"} equals
new { Key1=governorate.OptionValue, Key2= governorate.OptionListName  }
into groups
from g in groups.DefaultIfEmpty()
select new {
             person = person, 
             governorates = g.OptionName
           }

这将返回一个对象集合，每个对象包含一对person和governorate.OptionName。

来自MSDN

的第二个查询的一些解释

生成两个集合的左外连接的第一步是使用组连接执行内连接第二步是将第一个（左）集合的每个元素包含在结果集中，即使该元素在右集合中没有匹配项也是如此。这是通过在组连接的每个匹配元素序列上调用DefaultIfEmpty 来实现的。

Answer 2

from person in db.persons
join governorate in db.Options 
on   person.GovernorateId equals governorate.OptionValue  
where governorate.OptionListName =="Governorate"       
select person, governorate

OR 你可以试试这个

from person in db.persons
join governorate in db.Options.Where(x=>x.OptionListName =="Governorate") 
on   person.GovernorateId equals governorate.OptionValue 
select person, governorate

例如：我试过这个并且效果很好。

from u in User_Accounts.Where(x=>x.User_Account_Type =="Client")
join a in Clients on u.Client_ID equals a.Client_ID
select u

Answer 3

from p in db.persons
join h in db.Options on p.GovernorateId equals h.OptionValue && h.OptionListName equals "Governorate" into t
 from rt in t

 select new
 {
       p.person, h.governorate
 }).ToList();

在Linq加入条件

3 个答案: