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In LINQ, how can you execute multiple field joins in a single join?


I need to create a LINQ2DataSet query that joins multiple fields (as an example).

var result = from x in entity
join y in entity2 
       on x.field1 = y.field1 
          x.field2 = y.field2

I have yet to come across a viable solution (I can add the extra constraints to a where clause, but this is far from a suitable solution, or use this solution, but that assumes an equijoin).

Is it feasible to join on several fields in a single LINQ query?


var result = from x in entity
             join y in entity2
             on new { x.field1, x.field2 } equals new { y.field1, y.field2 }

is the solution I referenced as assuming an equijoin above.

Further EDIT

To respond to the complaint that my first example was an equijoin, I admit that it was. My current requirement is for an equijoin, and I have already used the approach I mentioned above.

However, I’m attempting to figure out what options and best practices I have / should use using LINQ. I’ll need to conduct a Date range query join with a table ID soon, and I was just anticipating the problem. It appears that I’ll have to include the date range in the where clause.

Thank you for all of your suggestions and comments, as usual.

Asked by johnc

Solution #1

var result = from x in entity
   join y in entity2 on new { x.field1, x.field2 } equals new { y.field1, y.field2 }

Answered by KristoferA

Solution #2

var result = from x in entity1
             join y in entity2
             on new { X1= x.field1, X2= x.field2 } equals new { X1=y.field1, X2= y.field2 }

If the column names in two entities disagree, you must do this.

Answered by RealNapster

Solution #3

The anonymous type solution should work just fine. With join clauses, LINQ can only describe equijoins, which is exactly what you’ve specified you want to express based on your original query.

If you don’t like the version with the anonymous type for some specific reason, you should explain that reason.

Please offer an example of what you truly want to do if you want to do something else than what you originally asked for.

EDIT: In response to the question’s edit, you must use a where clause instead of a join to make a “date range” join. It’s basically simply an issue of which optimizations are accessible because they’re semantically comparable. Equijoins (in LINQ to Objects, which includes LINQ to DataSets) provide easy optimization by making a lookup based on the inner sequence – think of it as a hashtable from key to a sequence of items matching that key.

It’s a little more difficult with date ranges. However, depending on what you mean by “date range join,” you might be able to do something similar – for example, if you’re planning on creating “bands” of dates (e.g. one per year) so that two entries that occur in the same year (but not on the same date) should match, you can do it simply by using that band as the key. If the join is more complicated, such as one side providing a range and the other providing a single date, matching if it falls within that range, I believe it would be better handled using a where clause (followed by a second from clause). You may do some extremely unusual magic if you

Answered by Jon Skeet

Solution #4

To finish, here’s an equivalent method chain syntax:

entity.Join(entity2, x => new {x.Field1, x.Field2},
                     y => new {y.Field1, y.Field2}, (x, y) => x);

The last argument (x, y) => x is what you choose (in this case, we choose x).

Answered by niieani

Solution #5

I believe that using the Where function is a more legible and versatile option:

var result = from x in entity1
             from y in entity2
                 .Where(y => y.field1 == x.field1 && y.field2 == x.field2)

This also makes it simple to go from an inner to a left join by adding. DefaultIfEmpty().

Answered by Alexei – check Codidact

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