從不同的表中取數據並將其顯示在查看索引中

Question

我有2個表Job_table和Employee_table。我想在Employee_table的索引視圖中顯示來自Job_table的emp_id和來自Employee_table的對應emp_name。

對於這個問題，我創建一個ViewModel EmployeeViewModel並將我的行爲結果的索引視圖與IEnumerable<EmployeeViewModel>綁定。對於EmployeeViewModel的定義是這樣的：

public class EmployeeViewModel 
{ 
    public int EmployeeId { get; set; } 
    public string EmployeeName { get; set; } 
    public string JobName { get; set; } 
    //..Other memberVariables.. 
} 

我的模型：

public class Employee 
    { 
     [Key] 
     [DatabaseGenerated(DatabaseGeneratedOption.Identity)] 
     public int EmployeeId { get; set; } 
     public string EmployeeName { get; set; } 
     public string Address { get; set; } 
     public virtual ICollection<Job> Jobs { get; set; } 
    } 

，工作臺，更名爲工作爲自己的方便：

public class Job 
{ 
    [Key] 
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)] 
    public int JobId { get; set; } 
    public string JobName { get; set; } 
    public JobCategory JobCategory { get; set; } 
    public int EmployeeId { get; set; } 
    public virtual ICollection<Employee> Employees { get; set; } 
} 

在我的索引行動，我通過連接兩個表創建結果集，將其綁定到IEnumerable<EmployeeViewModel>並將其作爲模型傳遞給視圖。該視圖應接收IEnumerable<EmployeeViewModel>正如我前面提到類型的模型，所以我需要查詢我的實體應該是這樣的：

public ActionResult Index() 
{ 
    //..something like this..this is IQueryable.. 
    //...convert this to IEnumerable and send this as the model to .. 
    //..the Index View as shown below..here you are querying your own tables, 
    //.. Employee and Job,and binding the result to the EmployeeViewModel which 
    //.. is passed on to the Index view. 
    IEnumerable<EmployeeViewModel> model=null; 
    model = (from c in db.Employees 
       join q in db.Jobs on c.EmployeeId equals q.EmployeeId 
       from q in jobs.DefaultIfEmpty() 
       group new { q, c } by c into grp 
       select new EmployeeViewModel 
       { 
        EmployeeId = grp.Key.EmployeeId, 
        EmployeeName = grp.Key.EmployeeName, 

       }); 

    return View(model); 
} 

我想顯示類似的結果：

|Employee 1| 
|----------| 
| Job 1 | 
| Job 2 | 

|Employee 2| 
|----------| 
| Job 3 | 
| Job 4 | 
| Job 5 | 

但現在我的結果是

|Employee 1| |Employee 1| 
|----------| |----------| 
| Job 1 | | Job 2 | 

|Employee 2| |Employee 2| |Employee 2| 
|----------| |----------| |----------| 
| Job 3 | | Job 4 | | Job  | 

如何刪除重複項？

Answer 1

這裏您需要的是在數據庫世界中稱爲LEFT JOIN。在SQL中如何連接兩個表有很多種可能性。每個變體都提供了不同的結果。默認JOIN是INNER連接，這意味着結果集中的每個實體都存在於兩個表中。 左連連接意味着實體可能存在於兩個表中，但是不可以。如果它在第二個表中不存在，則返回一個空的結果集。兩個表人和工作一個例子：

|Table Person | |Table Job  | 
| Id | Name | | Id | Job  | 
|----|---------| |----|-----------| 
| 1 | John | | 1 | Astronaut | 
| 2 | William | | 3 | Cashier | 

默認LINQ連接是INNER加入：

var r = from p in Person 
      join j in Jobs on p.Id equals j.Id 
      select ... 

將提供以下結果集：

| INNER JOIN RESULT | 
| Id | Name | Job  | 
|----|------|-----------| 
| 1 | John | Astronaut | 

The LEFT加入看起來在LINQ是這樣的：

var r = from p in Person 
      join j in Jobs on p.Id equals j.Id into jobs 
      from subjob in jobs.DefaultIfEmpty() 
      select ... 
       Job = subjob != null ? subjob.Job : "no job" 
       ... 

而結果現在設置如下：

| LEFT JOIN RESULT  | 
| Id | Name | Job  | 
|----|---------|-----------| 
| 1 | John | Astronaut | 
| 2 | William | no job | 

一個良好的視覺可以解釋本文A visual explanation of SQL joins中找到。

通常，LINQ中的默認連接是INNER JOIN，所以只有匹配的實體（在兩個表中）纔會被選中。你可以做一個LEFT JOIN with LINQ using a second from with DefaultIfEmpty：

model = (from e in db.Employees 
      join j in db.Jobs on e.EmployeeId equals j.EmployeeId into jobs 
      from subjob in jobs.DefaultIfEmpty() 
      select new EmployeeViewModel 
      { 
       EmployeeId = e.EmployeeId, 
       EmployeeName = e.EmployeeName, 
       JobName = subjob != null ? subjob.JobName : "no job entry" 
      }).Distinct(); 

微軟的良好LINQ的介紹可以在MSDN上找到：101 LINQ samples。

---

集團通過看起來是這樣的：

model = (from e in db.Employees 
      join j in db.Jobs on e.EmployeeId equals j.EmployeeId into jobs 
      from subjob in jobs.DefaultIfEmpty() 
      group e by e.EmployeeName into ge 
      select new 
      { 
       Key = ge.Key, 
       Data = ge 
      }); 

foreach (var groupItem in model) 
{ 
    foreach (var employeeViewModel in groupItem.Data) 
    { 
     // ... 
    } 
}