join clause (C# Reference)

The join clause is useful for associating elements from different source sequences that have no direct relationship in the object model. The only requirement is that the elements in each source share some value that can be compared for equality. For example, a food distributor might have a list of suppliers of a certain product, and a list of buyers. A join clause can be used, for example, to create a list of the suppliers and buyers of that product who are all in the same specified region.

A join clause takes two source sequences as input. The elements in each sequence must either be or contain a property that can be compared to a corresponding property in the other sequence. The join clause compares the specified keys for equality by using the special equals keyword. All joins performed by the join clause are equijoins. The shape of the output of a join clause depends on the specific type of join you are performing. The following are three most common join types:

• Inner join

• Group join

• Left outer join

Inner join

The following example shows a simple inner equijoin. This query produces a flat sequence of "product name / category" pairs. The same category string will appear in multiple elements. If an element from categories has no matching products, that category will not appear in the results.

var innerJoinQuery =
    from category in categories
    join prod in products on category.ID equals prod.CategoryID
    select new { ProductName = prod.Name, Category = category.Name }; //produces flat sequence

For more information, see Perform inner joins.

Group join

A join clause with an into expression is called a group join.

var innerGroupJoinQuery =
    from category in categories
    join prod in products on category.ID equals prod.CategoryID into prodGroup
    select new { CategoryName = category.Name, Products = prodGroup };

A group join produces a hierarchical result sequence, which associates elements in the left source sequence with one or more matching elements in the right side source sequence. A group join has no equivalent in relational terms; it is essentially a sequence of object arrays.

If no elements from the right source sequence are found to match an element in the left source, the join clause will produce an empty array for that item. Therefore, the group join is still basically an inner-equijoin except that the result sequence is organized into groups.

If you just select the results of a group join, you can access the items, but you cannot identify the key that they match on. Therefore, it is generally more useful to select the results of the group join into a new type that also has the key name, as shown in the previous example.

You can also, of course, use the result of a group join as the generator of another subquery:

var innerGroupJoinQuery2 =
    from category in categories
    join prod in products on category.ID equals prod.CategoryID into prodGroup
    from prod2 in prodGroup
    where prod2.UnitPrice > 2.50M
    select prod2;

For more information, see Perform grouped joins.

Left outer join

In a left outer join, all the elements in the left source sequence are returned, even if no matching elements are in the right sequence. To perform a left outer join in LINQ, use the DefaultIfEmpty method in combination with a group join to specify a default right-side element to produce if a left-side element has no matches. You can use null as the default value for any reference type, or you can specify a user-defined default type. In the following example, a user-defined default type is shown:

var leftOuterJoinQuery =
    from category in categories
    join prod in products on category.ID equals prod.CategoryID into prodGroup
    from item in prodGroup.DefaultIfEmpty(new Product { Name = String.Empty, CategoryID = 0 })
    select new { CatName = category.Name, ProdName = item.Name };

For more information, see Perform left outer joins.