我需要帮助在linq语句中执行左连接。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。我也意识到像我这样的问题很多，但我似乎无法将任何解决方案应用到我的案例中。
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尝试将此返回。
+---+------------+-------------+
| |Type |Count |
+---+------------+-------------+
| 1 | 1 | 1 |
| 3 | 3 | 1 |
+---+------------+-------------+
Linq声明。
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();
我尝试过做一些defaultifempty，但似乎无法使其发挥作用。
答案 0 :(得分：2)
这是MSDN链接＆＃34;如何：执行左外连接＆＃34;使用LINQ：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();
答案 1 :(得分：2)
哇 ..最后我做到了..
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();
答案 2 :(得分：1)
我无法在查询语法中执行此操作，但使用扩展方法语法将是
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);
}
使用适当的sql函数调用替换Int32.Parse以获取实际的DbContext查询代码。
使用查询语法，这可能是我能提出的最佳建议：
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()
};
答案 3 :(得分：1)
基本上您需要遵循join clause (C# Reference)中描述的左连接模式。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)
重要的是不在加入之后将其作为where条包含在内。
处理它的一种可能方法是：
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();
或者只需将其作为where子句应用于之前：
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）字段进行分组（就像在原始SQL查询中一样），因此以string键结束。如果您希望获得int密钥，则无法在db查询中执行此操作，因此您需要使用临时投影，上下文切换和最终投影，如上所示。
更新：我最初没有意识到的最后一件事是原始SQL Count(Pro.transportId)从联接的右侧排除NULL s。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();