Question

我有这样的表结构。

ContinentTable =＆gt; CountryTable =＆gt; UserTable

用户有CountryId。国家有ContinentId。

我想查询所有大陆的每个用户数。

我是通过SQL实现的。但我想使用linq扩展方法，因为我正在使用EF Core。

这是我的SQL查询

SELECT Continent.Id, Continent.Name, COUNT(Usr.UserName) AS 'User Count'
FROM Continents Continent, Countries Country, Users Usr 
WHERE Continent.Id = Country.ContinentId
AND Country.Id = Usr.CountryId
GROUP BY Continent.Id, Continent.Name

Answer 1

如果这是您的对象模型：

Dim continents As Dictionary(Of String, Dictionary(Of String, List(Of String)))

    continents = New Dictionary(Of String, Dictionary(Of String, List(Of String))) From
        {
            {"Europe", New Dictionary(Of String, List(Of String)) From
                {
                    {"France", New List(Of String) From {"Antoine", "Serge"}},
                    {"Germany", New List(Of String) From {"Hans", "Wolfgang"}},
                    {"Spain", New List(Of String) From {"Pablo", "Pedro"}}
                }
            },
            {"America", New Dictionary(Of String, List(Of String)) From
                {
                    {"USA", New List(Of String) From {"Kevin", "Caleb"}},
                    {"Mexico", New List(Of String) From {"Gustavo", "Oscar"}},
                    {"Brazil", New List(Of String) From {"Alexandro"}}
                }
            }
        }

然后这将是您的LINQ查询：

Dim result As IEnumerable(Of KeyValuePair(Of String, Integer)) = continents.Select(Of KeyValuePair(Of String, Integer))(Function(x As KeyValuePair(Of String, Dictionary(Of String, List(Of String)))) New KeyValuePair(Of String, Integer)(x.Key, x.Value.Select(Of Integer)(Function(y As KeyValuePair(Of String, List(Of String))) y.Value.Count).Sum))

Linq中的嵌套查询

1 个答案: