LINQ to SQL: Left join on multiple columns

Question

First of all, I have searched google/SO, checked some examples, but I didn't manage to write the proper linq expression:

This is how my working sql query looks like:

select *
from Places p
left join VoteLog v
on p.Id = v.PlaceId
and v.UserId = '076a11b9-6b14-4230-99fe-28aab078cefb' --demo userid

This is my attempt with linq:

public IQueryable<Place> GetAllPublic(string userId)
{
    var result = (from p in _db.Places
                 join v in _db.VoteLogs
                 on p.Id equals v.PlaceId // This works but doesn't fully reproduce my SQL query
                 // on new { p.Id, userId} equals new {v.PlaceId, v.UserId} -> Not ok
                 where p.Public == 1
                 select new
                 {
                    Id = p.Id,
                    UserId = p.UserId,
                    X = p.X,
                    Y = p.Y,
                    Titlu = p.Titlu,
                    Descriere = p.Descriere,
                    Public = p.Public,
                    Votes = p.Votes,
                    DateCreated = p.DateCreated,
                    DateOccured = p.DateOccured,
                    UserVoted = v.Vote
                 })
        .ToList()
        .Select(x => new Place()
        {
            Id = x.Id,
            UserId = x.UserId,
            X = x.X,
            Y = x.Y,
            Titlu = x.Titlu,
            Descriere = x.Descriere,
            Public = x.Public,
            Votes = x.Votes,
            DateCreated = x.DateCreated,
            DateOccured = x.DateOccured,
            UserVoted = x.UserVoted
        }).AsQueryable();

Try http://stackoverflow.com/questions/373541/how-to-do-joins-in-linq-on-multiple-fields-in-single-join — MichaelMao, Jul 18 '16 at 08:20

Gilad Green · Accepted Answer · 2016-07-18T09:02:12.377

18

In your query you didn't do any left join. Try this:

from p in _db.places
join v in _db.VoteLogs

//This is how you join by multiple values
on new { Id = p.Id, UserID = userId } equals new { Id = v.PlaceId, UserID = v.UserID } 
into jointData

//This is how you actually turn the join into a left-join
from jointRecord in jointData.DefaultIfEmpty()

where p.Public == 1
select new
{
    Id = p.Id,
    UserId = p.UserId,
    X = p.X,
    Y = p.Y,
    Titlu = p.Titlu,
    Descriere = p.Descriere,
    Public = p.Public,
    Votes = p.Votes,
    DateCreated = p.DateCreated,
    DateOccured = p.DateOccured,
    UserVoted = jointRecord.Vote 
    /* The row above will fail with a null reference if there is no record due to the left join. Do one of these:
       UserVoted = jointRecord ?.Vote - will give the default behavior for the type of Uservoted
       UserVoted = jointRecord == null ? string.Empty : jointRecord.Vote */
}

edited Jul 18 '16 at 09:02

answered Jul 18 '16 at 08:23

Gilad Green

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Error 5 The type of one of the expressions in the join clause is incorrect. Type inference failed in the call to 'GroupJoin'. – Gerald Hughes Jul 18 '16 at 08:32
@Stefan - It was because the type/names of the anonymous types of the `on` didn't match. Try now – Gilad Green Jul 18 '16 at 08:36
At UserVoted = v.Vote : Error 5 The name 'v' does not exist in the current context – Gerald Hughes Jul 18 '16 at 08:45
@Stefan - true - my bad - it was supposed to be `jointRecord.Vote` instead. Fixed – Gilad Green Jul 18 '16 at 09:02
Thank you very much Gilad Green :) – Gerald Hughes Jul 18 '16 at 09:08
how do I add a second set of field relation joins simulating an Or SQL condition – Golden Lion Nov 15 '21 at 22:27

Zein Makki · Answer 2 · 2016-07-18T08:31:33.753

You have to use .DefaultIfEmpty() to perform a left join. Then you need to decide what to do in case the right table produces null. You can use a ternary operator ( ? : ) for that.

var result = 
    (from p in _db.Places
    join v in _db.VoteLogs
    on new { p.Id, userId } equals new { v.PlaceId, v.UserId } into LEFTJOIN
    from result in LEFTJOIN.DefaultIfEmpty()
    where p.Public == 1
    select new
    {
        Id = p.Id,
        UserId = p.UserId,
        X = p.X,
        Y = p.Y,
        Titlu = p.Titlu,
        Descriere = p.Descriere,
        Public = p.Public,
        Votes = p.Votes,
        DateCreated = p.DateCreated,
        DateOccured = p.DateOccured,
        UserVoted = result == null ? null /* replace with your value */ : x.Vote
    }).AsQueryable();

return result;

score 1 · Answer 3 · answered Jul 18 '16 at 08:25

If your problem is assigning keywords to those multiple join conditions:

// on new { p.Id, userId} equals new {v.PlaceId, v.UserId}

try with

on new { a = p.Id, b = userId} equals new { a = v.PlaceId, b = v.UserId}

LINQ to SQL: Left join on multiple columns

3 Answers3

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