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How to do joins in LINQ on multiple fields in single join

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c#

join

linq

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How do I join two queries in LINQ?

LINQ Join queries. As we know the JOIN clause is very useful when merging more than two table or object data into a single unit. It combines different source elements into one and also creates the relationship between them. Using the join, you can grab the data based on your conditions.

Can we do Joins in LINQ?

In a LINQ query expression, join operations are performed on object collections. Object collections cannot be "joined" in exactly the same way as two relational tables. In LINQ, explicit join clauses are only required when two source sequences are not tied by any relationship.

What is group join in LINQ?

The group join is useful for producing hierarchical data structures. It pairs each element from the first collection with a set of correlated elements from the second collection. For example, a class or a relational database table named Student might contain two fields: Id and Name .

Can I join a table to a list using LINQ?

When the query actually runs, LINQ should be able to create a temp table or table variable with the data from the local list and then join on that. This is a feature that should absolutely be included in the framework.


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

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 }

You need to do this, if the column names are different in two entities.


The solution with the anonymous type should work fine. LINQ can only represent equijoins (with join clauses, anyway), and indeed that's what you've said you want to express anyway 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.

If you want to do something other than what you originally asked for, please give an example of what you really want to do.

EDIT: Responding to the edit in the question: yes, to do a "date range" join, you need to use a where clause instead. They're semantically equivalent really, so it's just a matter of the optimisations available. Equijoins provide simple optimisation (in LINQ to Objects, which includes LINQ to DataSets) by creating a lookup based on the inner sequence - think of it as a hashtable from key to a sequence of entries matching that key.

Doing that with date ranges is somewhat harder. However, depending on exactly what you mean by a "date range join" you may be able to do something similar - if you're planning on creating "bands" of dates (e.g. one per year) such that two entries which occur in the same year (but not on the same date) should match, then you can do it just by using that band as the key. If it's more complicated, e.g. one side of the join provides a range, and the other side of the join provides a single date, matching if it falls within that range, that would be better handled with a where clause (after a second from clause) IMO. You could do some particularly funky magic by ordering one side or the other to find matches more efficiently, but that would be a lot of work - I'd only do that kind of thing after checking whether performance is an issue.


Just to complete this with an equivalent method chain syntax:

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

While the last argument (x, y) => x is what you select (in the above case we select x).


I think a more readable and flexible option is to use Where function:

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

This also allows to easily change from inner join to left join by appending .DefaultIfEmpty().


var result = from x in entity
             join y in entity2
             on new { X1= x.field1, X2= x.field2 } equals new { X1=y.field1, X2= y.field2 }
             select new 
             {
               /// Columns
              };

you could do something like (below)

var query = from p in context.T1

        join q in context.T2

        on

        new { p.Col1, p.Col2 }

        equals

         new { q.Col1, q.Col2 }

        select new {p...., q......};