LINQ的结合左联接数据

说我有以下数据库：LINQ的结合左联接数据

Users 
------- 
UserId (PK) 
UserName 

Roles 
----- 
RoleId (PK) 
RoleName 

UserRoles 
--------- 
UserId (PK) 
RoleId (PK)

用户1-M的UserRole M-1作用

使用LinqToSQL，我想回到下面的一组：

[User1], [Role1, Role2, Role3] 
[User2], [Role2, Role3] 
[User3], []

等......

什么是创建此LinqToSql查询的最有效方法？

另外，如果我想创建一个过滤器来仅返回具有Role1的用户，那会带来什么？

Thx。

来源

2009-09-28 zzz

定义“高效”。但除此之外...

from u in dataContext.Users 
select new { User = u, Roles = u.UserRoles.Select(ur => ur.Role) }

和过滤用户可以通过RoleID：

from u in dataContext.Users 
where u.UserRoles.Any(ur => ur.RoleID == 1) 
select u

或其他一些Role属性，比如说，Name：

from u in dataContext.Users 
where u.UserRoles.Any(ur => ur.Role.Name == "Role 1") 
select u

结合一起：

from u in dataContext.Users 
select new 
{ 
    User = u, 
    Roles = from ur in u.UserRoles 
      where ur.RoleID == 1 || ur.Role.Name == "Role 1" 
      select ur.Role 
}

来源

2009-09-28 17:56:11

我并不熟悉LINQ to SQL，但我不相信这是正确的。 'u.UserRoles'不会提示'UserRoles'是用户表中的一个字段？ – Tinister 2009-09-28 18:01:28

完全没有。当另一个表通过外键引用此表时，L2S会自动创建集合类型的“导航属性”。只要'UserRole'表的'UserID'设置为'User'的FK，为'User'生成的实体类将拥有一个名为'UserRoles'的属性。 – 2009-09-28 18:33:07

我将如何过滤角色ID？ – zzz 2009-09-28 20:36:51

这是我将构建以获得您想要的结果集一下子

from u in Users 
join ur in UserRoles on u.UserId equals ur.UserId 
join r in Roles on ur.RoleId equals r.RoleId 
group r by u into grouping 
select grouping

它产生下面的SQL单查询：

SELECT [t0].[UserId], [t0].[Username] 
FROM [Users] AS [t0] 
INNER JOIN [UserRoles] AS [t1] ON [t0].[UserId] = [t1].[UserId] 
INNER JOIN [Roles] AS [t2] ON [t1].[RoleId] = [t2].[RoleId] 
GROUP BY [t0].[UserID], [t0].[Username] 
GO 

-- Region Parameters 
DECLARE @x1 Int = 2 
-- EndRegion 
SELECT [t2].[RoleId], [t2].[RoleName] 
FROM [Users] AS [t0] 
INNER JOIN [UserRoles] AS [t1] ON [t0].[UserId] = [t1].[UserId] 
INNER JOIN [Roles] AS [t2] ON [t1].[RoleId] = [t2].[RoleId] 
WHERE @x1 = [t0].[UserId]

@Pavel的看起来像它产生更好的SQL声明：

SELECT [t0].[UserId], [t0].[Username], [t2].[RoleId], [t2].[RoleName] (
    SELECT COUNT(*) 
    FROM [UserRoles] AS [t3] 
    INNER JOIN [Roles] AS [t4] ON [t4].[RoleId] = [t3].[RoleId] 
    WHERE [t3].[UserId] = [t0].[UserId] 
    ) AS [value] 
FROM [Users] AS [t0] 
LEFT OUTER JOIN ([UserRoles] AS [t1] 
    INNER JOIN [Roles] AS [t2] ON [t2].[RoleId] = [t1].[RoleId]) ON [t1].[UserId] = [t0].[UserId] 
ORDER BY [t0].[UserId], [t1].[UserRoleId], [t2].[RoleId]

就效率而言，测试将是找出最适合您的最高性能方法的最佳方法情况。

来源

2009-09-28 18:08:39 bdukes

LINQ的结合左联接数据

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