LINQ to SQL - Left Outer Join with multiple join conditions

user_30639 0
Viewed 158674

I have the following SQL, which I am trying to translate to LINQ:

SELECT f.value
FROM period as p 
LEFT OUTER JOIN facts AS f ON p.id = f.periodid AND f.otherid = 17
WHERE p.companyid = 100

I have seen the typical implementation of the left outer join (ie. into x from y in x.DefaultIfEmpty() etc.) but am unsure how to introduce the other join condition (AND f.otherid = 17)

EDIT

Why is the AND f.otherid = 17 condition part of the JOIN instead of in the WHERE clause? Because f may not exist for some rows and I still want these rows to be included. If the condition is applied in the WHERE clause, after the JOIN - then I don't get the behaviour I want.

Unfortunately this:

from p in context.Periods
join f in context.Facts on p.id equals f.periodid into fg
from fgi in fg.DefaultIfEmpty()
where p.companyid == 100 && fgi.otherid == 17
select f.value

seems to be equivalent to this:

SELECT f.value
FROM period as p 
LEFT OUTER JOIN facts AS f ON p.id = f.periodid 
WHERE p.companyid = 100 AND f.otherid = 17

which is not quite what I'm after.

outer-join linq-to-sql sql linq c#
7 Answers

You need to introduce your join condition before calling DefaultIfEmpty(). I would just use extension method syntax:

from p in context.Periods
join f in context.Facts on p.id equals f.periodid into fg
from fgi in fg.Where(f => f.otherid == 17).DefaultIfEmpty()
where p.companyid == 100
select f.value

Or you could use a subquery:

from p in context.Periods
join f in context.Facts on p.id equals f.periodid into fg
from fgi in (from f in fg
             where f.otherid == 17
             select f).DefaultIfEmpty()
where p.companyid == 100
select f.value
user_542490

this works too, ...if you have multiple column joins

from p in context.Periods
join f in context.Facts 
on new {
    id = p.periodid,
    p.otherid
} equals new {
    f.id,
    f.otherid
} into fg
from fgi in fg.DefaultIfEmpty()
where p.companyid == 100
select f.value
user_12749560

Can be written using composite join key. Also if there is need to select properties from both left and right sides the LINQ can be written as

var result = context.Periods
    .Where(p => p.companyid == 100)
    .GroupJoin(
        context.Facts,
        p => new {p.id, otherid = 17},
        f => new {id = f.periodid, f.otherid},
        (p, f) => new {p, f})
    .SelectMany(
        pf => pf.f.DefaultIfEmpty(),
        (pf, f) => new MyJoinEntity
        {
            Id = pf.p.id,
            Value = f.value,
            // and so on...
        });
user_2094590

Whilst my response below does not directly answer the question, I believe it offers an alternative to the core issue that a read might find valuable.

I end up at this thread and others looking for the EF equivalent of a simple self join SQL I had written. I included Entity Framework in my project to make my DB interactions easier, but having to use "GroupJoin" , "SelectMany" and "DefaultIfEmpty" is like having to translate to another language.

Additionally I work with engineer who are excellent at SQL but have limited C# skills. So I want a solution that they could read.

The solution that worked for me was:

context.Database.SqlQuery<Class>

This allows executing of SQL commands with result return in a typed object. So long as the column names returned match the property names of the given Class. For example:

 public class MeasurementEvent
{
    public int ID { get; set; }
    public string JobAssemID { get; set; }
    public DateTime? InspDate { get; set; }


}

var list = context.Database.SqlQuery<MeasurementEvent>(@"
                Select op.umeMeasurementEventID as ID, op.umeJobID+'.'+Cast(op.umeAssemblyID as varchar) as JobAssemID ,  insp.umeCreatedDate as InspDate 
                from uMeasurementEvents as op 
                    left JOIN   uMeasurementEvents as insp on op.umeJobID = insp.umeJobID and op.umeAssemblyID = insp.umeAssemblyID and insp.umeInstanceId = 1 and insp.umeIsInspector = 1
                  where  op.umeInstanceId = 1  and op.umeIsInspector = 0")
            .ToList();
user_36358750
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