LINQ查询表达式中的子查询

Question

Below is my sql query:

select e.id, m.id, e.Name, (select 1 where e.Id Is Null) as NotExists
    from    Employee e 
            INNER JOIN Department d
            ON e.Id = d.Id  
            LEFT JOIN Manager m
            ON e.Id = m.Id
            order by e.Name

I am trying to write above query into LINQ like this:

var result = from e in employeeRep
             join d in departmentRep
             on e.Id equals d.Id
             join m in manager
             on e.Id = m.Id
             order by e.Name
             select new
             {
               Name = e.Name,
               EmpId = e.Id,
               ManagerId = m.Id,
               DepartmentId = d.Id,
               DeparmentName = d.Name,
               NotExists = // here I want to write this query (select 1 where e.Id Is Null) as NotExists
             } ;

can anybody tell me how to achieve this

Thanks

Answer 1

If Id is nullable then you can do this:

select new
             {
               Name = e.Name,
               EmpId = e.Id,
               ManagerId = m.Id,
               DepartmentId = d.Id,
               DeparmentName = d.Name,
               NotExists = e.Id == null ? 1 : 0 

            }

Answer 2

You can use the let keyword. It allows you to create a subqueries, assign them a name, and then use them later in your code. Although, it's more helpful in much complex subqueries it will work here as well.

var result = from e in employeeRep
         join d in departmentRep
         on e.Id equals d.Id
         join m in manager
         on e.Id = m.Id
         order by e.Name
         let mySub = // write your query here.
         select new
         {
           Name = e.Name,
           EmpId = e.Id,
           ManagerId = m.Id,
           DepartmentId = d.Id,
           DeparmentName = d.Name,
           NotExists = mySub
         };