Linq to sql 中如何进行 left join

可以用 into 语法，配合 DefaultIfEmpty() 方法加载右侧表中的空值。
例子：

var query =
    from r in db.Roles
    join sr in
        (
        from ss in db.StaffInRoles
where ss.StaffId == staffId
        select ss
        )
        on r.Id equals sr.RoleId
into x
from y in x.DefaultIfEmpty()
    select new RoleInfo
    {
        RoleId = r.Id,
        RoleName = r.Name,
        IsSet = y != null
    };

注意这里最后可以通过 y != null 来判断右值是否为空。
这个 Linq to sql 语句生成的对应 SQL 为：

exec sp_executesql N'SELECT [t0].[Id] AS [RoleId], [t0].[Name] AS [RoleName],
    (CASE
        WHEN [t2].[test] IS NOT NULL THEN 1
        ELSE 0
     END) AS [IsSet]
FROM [Role] AS [t0]
LEFT OUTER JOIN (
    SELECT 1 AS [test], [t1].[StaffId], [t1].[RoleId]
    FROM [StaffInRole] AS [t1]
    ) AS [t2] ON ([t0].[Id] = [t2].[RoleId]) AND ([t2].[StaffId] = @p0)',N'@p0 int',@p0=1

over.