通过过滤将SQL连接转换为带有EF的LINQ

Question

I'm learning Entity Framework and attempting to convert an existing SQL query to a LINQ query, but struggling to convert it.

SELECT taskItems.Description,taskItemResponses.IsCompleted,TaskItemResponses.userId,TaskItemResponses.Notes 
FROM TaskLists
LEFT JOIN TaskItems ON TaskLists.TaskListId = TaskItems.TaskListId
LEFT JOIN TaskItemResponses ON TaskItemResponses.TaskItemId = TaskItems.TaskItemId 
AND TaskItemResponses.UserId = '1'

This works fine for me, it brings back the following data, always showing the list of Tasks, and if a user has responded to any of them, if they've completed it and what notes they've added.

Description  IsCompleted   userId  Notes
Task A       NULL          NULL    NULL
Task B       NULL          NULL    NULL
Task C       NULL          NULL    NULL
Task D       1             1       I've done this now.
Task E       NULL          NULL    NULL

But when I'm trying to convert this to a LINQ query within C# I can't figure out the syntax, so far i've got

var query = from t in DbContext.TaskList
join ti in DbContext.TaskItem on t.TaskListId equals ti.TaskListId
join tr in DbContext.TaskItemResponse on ti.TaskItemId equals tr.TaskItemId into trj
from x in trj.DefaultIfEmpty()
where x.UserId == userId
select t;

But this isn't filtering for a particular UserId, and instead returns

Description  IsCompleted   userId  Notes
Task A       0             2       Great
Task B       1             2       Okay
Task C       1             3       Nope
Task D       1             1       I've done this now.
Task E       0             5       Ok. 

Answer 1

See that. May be you need ome minor changes.

var query = from t in DbContext.TaskList
    join ti in DbContext.TaskItem on t.TaskListId equals ti.TaskListId
    join tr in DbContext.TaskItemResponse on ti.TaskItemId equals tr.TaskItemId
    where(tr.UserId == '1')
    select new tempObjList
    {
        Description = taskItems.Description,
        IsCompleted = taskItemResponses.IsCompleted,
        userId = TaskItemResponses.userId,
        Notes = TaskItemResponses.Notes
    };

make "tempObjList" model class.

Answer 2

The correct way to convert SQL LEFT JOIN with right side filter to LINQ is to apply the right side filter before the join operator.

Here is the LINQ equivalent of your SQL query (of course you can correct the field names/types if needed):

var query = 
    from t in DbContext.TaskList
    join ti in DbContext.TaskItem on t.TaskListId equals ti.TaskListId
    into tij from ti in tij.DefaultIfEmpty() // left join
    join tr in DbContext.TaskItemResponse.Where(x => x.UserId == userId) // filter
        on ti.TaskItemId equals tr.TaskItemId
    into trj from tr in trj.DefaultIfEmpty() // left join
    select new
    {
        ti.Description,
        IsCompleted = (bool?)tr.IsCompleted,
        userId = (int?)tr.userId,
        tr.Notes 
    };

Answer 3

在您的EF代码中，第一个联接是INNER联接，第二个联接是LEFT OUTER联接

Answer 4

Have a look at these:

• First Option
• Second Option

Hope these could save you some time.