LINQ: Self join query, how to accomplish this?

问题

Can anyone help?

I have 1 class, basically it holds Members and within that class is a List.

The members i have in a List also... So basically it goes like this,

I have 2 members and each member has a number of sessions.

I wish to only return each member with 1 Session.

I have done a LINQ query, but of course it doesn't work...

I think i need to do a self join, any ideas?

Basically my error is m doesn't exist in my subquery self join.

var sessions =  
from m in this.members                     
join s in
(
    from se in m.Sessions
    group se by se.Name into g
    select new {Name = g.Key, SessioEndTime = g.Max(a=>a.SessioEndTime)}
)   
on m.Name equals s.Name                    
select new { MemberName = m.Name, SessionTime = s.SessioEndTime}

I would appreciate any feedback anyone has.

Thanks in advance.

EDIT

Ok i managed to do it like the following, but is this the best way?

var sessions =  
from m in this.members                     
let sn = m.Sessions.OrderByDescending(a => a.SessionEndTime).FirstOrDefault()                
select new { MemberName = m.Name, SessionTime = sn.SessioEndTime}

This way sn contains 1 record, but i have access to all the properties...

But is this the best way to do using a LET?

Thanks.

回答1:

Unless I am missing something you need this, no?

var sessions = 
   from m in members
   select new { 
      MemberName = m.Name, 
      SessionTime = m.Sessions.Max(s => s.SessioEndTime)
   };

You have to change the way you think about LINQ queries, think more from object point rather than from SQL implementation point. What is it that I need? I need all members, each with its latest session end time, then act on that.

EDIT: The let option you used is ok, just keep something in mind FirstOrDefault will return null if member has an empty list of Sessions, and then sn.SessionEndTime hits null reference. If on the other hand you are certain that every member has at least one session use First instead or aggregate. Also don't use FirstOrDefault() in the let, it kind of messes up the LINQ and prevents it from tying it to the master (causing a separate SQL query for each master to detect missing subsets), so usable queries with let are:

from m in Members                     
let sn = m.Sessions.Max(s => s.SessioEndTime)                
select new { MemberName = m.Name, SessionTime = sn};

from m in Members                     
let sn = m.Sessions.OrderByDescending(a => a.SessioEndTime).First()              
select new { MemberName = m.Name, SessionTime = sn.SessioEndTime};

As for ordering vs Max aggregation, both queries will generate a subquery:

-- MAX    
SELECT [t0].[Name] AS [MemberName], (
    SELECT MAX([t1].[SessioEndTime])
    FROM [Session] AS [t1]
    WHERE [t1].[memberId] = [t0].[id]
    ) AS [SessionTime]
FROM [Member] AS [t0]
GO

-- ordering
SELECT [t0].[Name] AS [MemberName], (
    SELECT [t2].[SessioEndTime]
    FROM (
        SELECT TOP (1) [t1].[SessioEndTime]
        FROM [Session] AS [t1]
        WHERE [t1].[memberId] = [t0].[id]
        ORDER BY [t1].[SessioEndTime] DESC
        ) AS [t2]
    ) AS [SessionTime]
FROM [Member] AS [t0]

With a descending index on SessioEndTime the ordering script is about twice slower (you can get execution plans for these to check for yourself), without the index its about 5times slower.

来源：https://stackoverflow.com/questions/5026970/linq-self-join-query-how-to-accomplish-this

标签

linq

linq-to-sql

linq-to-objects

self-join