Linq Left join, where, group by, count()

Question

我需要在 linq 语句中进行左连接的帮助。我的 T-sql 查询按预期工作，但我似乎无法从 Linq 获得想要的结果。我也意识到有很多像我这样的问题，但我似乎无法将任何解决方案应用于我的案例。

产品表

+---+------------+-----------+
|   |transportID |  Type(int)|
+---+------------+-----------+
| 1 | 5          | 1         |
| 2 | 5          | 3         |
| 3 | 6          | 3         |
+---+------------+-----------+

商店

+---+------------+-------------+
|   |Name        |Type1(string)|
+---+------------+-------------+
| 1 | Ho         | 1           |
| 2 | He         | 2           |
| 3 | Be         | 3           |
| 4 | Ke         | 4           |
| 5 | Fe         | 5           |
+---+------------+-------------+

我想要的结果是

+---+------------+-------------+
|   |Type        |Count        |
+---+------------+-------------+
| 1 | 1          | 1           |
| 2 | 2          | 0           |
| 3 | 3          | 1           |
| 4 | 4          | 0           |
| 5 | 5          | 0           |
+---+------------+-------------+

我的 tsql 按预期工作

SELECT 
    Type1,
    Count(Pro.transportId) as Count

FROM dbo.stores as sto

left Join dbo.products as pro on (sto.Type1 = pro.Type AND pro.transportId=5)

Where Type1 is not null
  group by Type1
  ORDER BY Type1 * 1 ASC

我的 Linq 尝试返回这个。

+---+------------+-------------+
|   |Type        |Count        |
+---+------------+-------------+
| 1 | 1          | 1           |
| 3 | 3          | 1           |
+---+------------+-------------+

Linq 语句。

var res =   (from sto in _context.Stores
                             join pro in _context.Products on sto.Type1 equals System.Data.Objects.SqlClient.SqlFunctions.StringConvert((double)pro.Type).Trim()
                             where pro.transportId == transportId
                             group pro by pro.Type1 into pt1
                             select new TypeTransportation()
                             {
                                 Type = pt1.Key, // Needs to be int
                                 Count = pt1.Count()
                             }).ToList();

我尝试过一些 defaultifempty 但似乎无法使其工作。

Answer 1

这是 MSDN 链接“如何：使用 LINQ 执行左外连接”：https://msdn.microsoft.com/en-gb/library/bb397895.aspx

你的代码应该是这样的：

        var res = (from sto in _context.Stores
               join pro in _context.Products on sto.Type1 equals System.Data.Objects.SqlClient.SqlFunctions.StringConvert((double)pro.Type).Trim() into grpJoin
               from product in grpJoin.DefaultIfEmpty()
               where product.transportId == transportId
               group product by product.Type1 into pt1
               select new TypeTransportation()
               {
                   Type = pt1.Key, // Needs to be int
                   Count = pt1.Count()
               }).ToList();

Answer 2

哇 ..我终于做到了..

      var transportId = 5;
      var res = from s in _context.Stores
                let Type = _context.Stores.Take(1).Select(x => s.Type1).Cast<int>().FirstOrDefault()
                group Type by Type into pt1
                select new TypeTransportation
                {
                    Type = pt1.Key, // Needs to be int
                    Count = _context.Products.Where(i => i.transportId == transportId && i.Type == pt1.Key).Count()
                };            

      foreach (var item in res)
      {
          Console.WriteLine(item.Type + " " + item.Count);
      }

      Console.ReadKey();

Answer 3

我不能在查询语法中做到这一点，但使用扩展方法语法它将是

var products = new[]
{
    new {transportId = 5, Type = 1},
    new {transportId = 5, Type = 3},
    new {transportId = 6, Type = 3},
    new {transportId = 5, Type = 3},
    new {transportId = 5, Type = 5},
};

var stores = new[]
{
    new {Name = "Ho", Type1 = "1"},
    new {Name = "He", Type1 = "2"},
    new {Name = "Be", Type1 = "3"},
    new {Name = "Ke", Type1 = "4"},
    new {Name = "Fe", Type1 = "5"},
};

var transportId = 5;
var res = stores                    
    .GroupJoin(
        inner: products
            .Where(product =>
                product.transportId == transportId),
        innerKeySelector: product => product.Type,
        outerKeySelector: store => Int32.Parse(store.Type1),
        resultSelector: (store, storeProducts) =>
            new
            {
                StoreType = store.Type1,
                StoreName = store.Name,
                ProductsCount = storeProducts.Count()
            })
    .ToList();

foreach (var item in res)
{
    Console.WriteLine(item);
}

只需将Int32.Parse 替换为实际 DbContext 查询代码的适当 sql 函数调用即可。

---

对于查询语法，这可能是我能提出的最好的建议：

var res =
    from store in stores
    join product in 
        (from prod in products where prod.transportId == transportId select prod)
        on store.Type1 equals product.Type.ToString() into storeProducts
    select new
    {
        StoreType = store.Type1,
        StoreName = store.Name,
        ProductsCount = storeProducts.Count()
    };

Answer 4

基本上你需要遵循join clause (C# Reference) 中描述的left join 模式。唯一棘手的部分是

中的pro.transportId=5 条件

left Join dbo.products as pro on (sto.Type1 = pro.Type AND pro.transportId=5)

重要的是不将其作为where 子句在加入之后。

一种可能的处理方式如下：

var res = (from sto in _context.Stores
           join pro in _context.Products
           on new { sto.Type1, transportId } equals
              new { Type1 = pro.Type.ToString(), pro.transportId }
           into storeProducts
           from pro in storeProducts.DefaultIfEmpty()
           group sto by sto.Type1 into pt
           select new 
           {
               Type = pt.Key, // the string value, there is no way to convert it to int inside the SQL
               Count = pt.Count()
           }).AsEnumerable() // switch to LINQ to Objects context
           .Select(pt => new TypeTransportation()
           {
               Type = Convert.ToInt32(pt.Type), // do the conversion here
               Count = pt.Count()
           }).ToList();

或者只是将其作为where 子句在加入之前应用：

var res = (from sto in _context.Stores
           join pro in _context.Products.Where(p => p.transportId == transportId)
           on sto.Type1 equals pro.Type.ToString()
           into storeProducts
           // the rest ... (same as in the first query)

要提到的另一个细节是，为了使LEFT JOIN 有效地应用，您需要按左表（在您的情况下为Stores）字段（就像在原始SQL 查询中一样）分组，从而以string 键。如果你想得到int 键，在db 查询中是没有办法的，所以你需要使用临时投影、上下文切换和最终投影，如上图所示。

更新：我最初没有意识到的最后一件事是原始 SQL Count(Pro.transportId) 将 NULLs 排除在连接的右侧。所以最终正确的等效 LINQ 查询是：

var res = (from sto in _context.Stores
           join pro in _context.MyProducts
           on new { sto.Type1, transportId } equals
              new { Type1 = pro.Type.ToString(), pro.transportId }
           into storeProducts
           from pro in storeProducts.DefaultIfEmpty()
           group new { sto, pro } by sto.Type1 into pt
           select new
           {
               Type = pt.Key,
               Count = pt.Sum(e => e.pro != null ? 1 : 0)
           })
           .AsEnumerable()
           .Select(pt => new TypeTransportation()
           {
               Type = Convert.ToInt32(pt.Type),
               Count = pt.Count
           }).ToList();