具有聯接和多個計數的C＃LINQ查詢

Question

我需要將此SQL查詢轉換為Linq-Entity查詢

SELECT Company.name, COUNT(DISTINCT User.id), COUNT(DISTINCT Office.id)
FROM Company
INNER JOIN Office ON Company.id = Office.companyId
INNER JOIN Employee ON Office.id = Employee.officeId
GROUP BY Company.name

因此，我想要一個能夠給我一個公司名稱，唯一雇員人數和一行辦公室人數的結果。

我有這些實體

public class Company
{
    public int id { get; set; }
    public string name { get; set; }
    public List<Office> offices { get; set; }
}

public class Office
{
    public int id { get; set; }
    public string name { get; set; }
    public int companyId { get; set; }
    public List<Employee> employees { get; set; }
}

public class Employee
{
    public int id { get; set; }
    public string name { get; set; }
    public int officeId { get; set; }
}

和ViewModel：

public class MyViewModel
{
    public Company company { get; set; }
    public int employeeCount { get; set; }
    public int officeCount { get; set; }
}

我在控制器中一直嘗試的操作：

var viewModel =
    from c in _context.Companies
    join o in _context.Offices on c.id equals o.companyId
    join e in _context.Employees on o.id equals e.officeId
    select new MyViewModel { company = c, employeeCount = ??, officeCount = 
    ??}

return View(viewModel);

所以我不知道count（）和group by如何工作。

Answer 1

首先，LINQ沒有SQL COUNT(DISTINCT expr)構造的直接等效項。

其次，更重要的是，在LINQ to Entities中，您不需要遵循SQL規則。 我們不使用聯接，而是使用導航屬性，基本上像使用對象一樣編寫查詢，然后讓EF將其轉換為SQL。

有問題的LINQ to Entities查詢是自然而簡單的，因為：

var query = _context.Companies
    .Select(c => new MyViewModel
    {
        company = c,
        officeCount = c.offices.Count(),
        employeeCount = c.offices.Sum(o => o.employees.Count()),
    });