SQL to Entity Framework，内部SELECT有多个JOIN

Question

我有以下SQL查询，我需要使用导航属性重写到Entity Framework。基本上，员工有许多人可以自愿做的任务。我试图找到至少有一名志愿者被分配到任何任务的所有员工。

SELECT j.noTasks, e.*
FROM Employee e
LEFT JOIN
(SELECT e.id, COUNT(vt.TaskId) as "noTasks"
FROM Employee e
LEFT JOIN Task t on e.id = t.EmployeeId
LEFT JOIN VolunteerTask vt on vt.TaskId = t.id
GROUP BY e.id) j
ON e.id = j.id AND noTasks > 0

麻烦的部分是内部的SELECT。我已经阅读了文档，但是我很难理解它。

员工类：

public partial class Employee : IEntity
{
    public int id { get; set; }

    [Required]
    [StringLength(100)]
    public string Name { get; set; }

    [StringLength(100)]
    public string WorkPlace { get; set; }

    public int EmployeeTypeId { get; set; }

    [Required]
    [StringLength(11)]
    public string OrganizationNumber { get; set; }

    [StringLength(20)]
    public string Telephone { get; set; }

    [Required]
    [StringLength(100)]
    public string StreetAddress { get; set; }

    [StringLength(200)]
    public string CoAddress { get; set; }

    [Required]
    [StringLength(20)]
    public string ZipCode { get; set; }

    [StringLength(200)]
    public string City { get; set; }

    [StringLength(200)]
    public string HomePage { get; set; }

    [Required]
    [StringLength(200)]
    public string ContactPerson { get; set; }

    [StringLength(30)]
    public string ContactPhone { get; set; }

    [StringLength(30)]
    public string ContactPhone2 { get; set; }

    [StringLength(200)]
    public string ContactEmailAddress { get; set; }

    public DateTime RegisteredDate { get; set; }

    public virtual EmployeeType EmployeeType { get; set; }

    [Required]
    public UserStatus Status { get; set; }

    public string ImageFileName { get; set; }
}

任务类：

public partial class Task : IEntity
{
    public Task()
    {
        VoluntaryTasks = new HashSet<VoluntaryTask>();
        TaskInterest = new HashSet<TaskInterest>();
        TaskCalendarItems = new HashSet<TaskCalendarItem>();
    }

    public int id { get; set; }

    [StringLength(100)]
    public string Name { get; set; }

    public int EmployeeId { get; set; }

    public int AreaId { get; set; }

    [Column(TypeName = "ntext")]
    public string Description { get; set; }

    public int NoOfVolunteers { get; set; }

    public DateTime? StartDate { get; set; }

    public DateTime? PublicationDate { get; set; }

    public DateTime EndDate { get; set; }

    [Required]
    [StringLength(200)]
    public string ContactPerson { get; set; }

    [Required]
    [StringLength(30)]
    public string ContactTelephone { get; set; }

    [StringLength(30)]
    public string ContactTelephone2 { get; set; }

    [StringLength(100)]
    public string EmailAddress { get; set; }

    public bool isPublished { get; set; }

    public bool NotificationMailSent { get; set; }

    public double? Latitude { get; set; }

    public double? Longitude { get; set; }

    public int Priority { get; set; }

    public RecordStatus RecordStatus { get; set; }

    public virtual Area Area { get; set; }  

    public string LocationDescription {get; set;}

    public virtual Employee Employee { get; set; }

    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly")]
    public virtual ICollection<VoluntaryTask> VoluntaryTasks { get; set; }

    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly")]
    public virtual ICollection<TaskInterest> TaskInterest { get; set; }
    public virtual ICollection<TaskLanguageSkill> TaskLanguageSkills { get; set; }

    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly")]
    public virtual ICollection<TaskCalendarItem> TaskCalendarItems { get; set; }
}

VoluntaryTask类：

public partial class VoluntaryTask
{
    //[Key]
    [Column(Order = 0)]
    [DatabaseGenerated(DatabaseGeneratedOption.None)]
    public int VoluntaryId { get; set; }

    //[Key]
    [Column(Order = 1)]
    [DatabaseGenerated(DatabaseGeneratedOption.None)]
    public int TaskId { get; set; }

    //[Key]
    public VoluntaryTaskStatus Status { get; set; }

    public DateTime StartDate { get; set; }

    public DateTime? EndDate { get; set; }

    public virtual Task Task { get; set; }

    public virtual Voluntary Voluntary { get; set; }
}