Cross join consists to perform a Cartesian product
of two sets or sequences. The following example shows a simple Cartesian product
of the sets A and B: A (a1, a2) B (b1, b2) => C (a1 b1, a1 b2, a2 b1, a2, b2 ) is the Cartesian product's result. Linq to Sql allows using Cross join operations. Cross join is not equijoin, means that no predicate expression
of equality in the Join clause
of the query. To define a cross join query, you can use multiple from clauses. Note that there's no explicit operator for the cross join. In the following example, the query must join a sequence
of Product with a sequence
of Pricing Rules: 1: //Fill the data source
2: var products = new List<Product>
3: {
4: new Product{ProductID="P01",ProductName="Amaryl"},
5: new Product {ProductID="P02", ProductName="acetaminophen"}
6: };
7:
8: var pricingRules = new List<PricingRule>
9: {
10: new PricingRule {RuleID="R_1", RuleType="Free goods"},
11: new PricingRule {RuleID="R_2", RuleType="Discount"},
12: new PricingRule {RuleID="R_3", RuleType="Discount"}
13: };
14:
15: //cross join query
16: var crossJoin = from p in products
17: from r in pricingRules
18: select new { ProductID = p.ProductID, RuleID = r.RuleID };
Below the definition
of the two entities using in the above example.
1: public class Product
2: {
3: public string ProductID { get; set; }
4: public string ProductName { get; set; }
5: }
1: public class PricingRule
2: {
3: public string RuleID { get; set; }
4: public string RuleType { get; set; }
5: }
Doing this:
1: foreach (var result in crossJoin)
2: {
3: Console.WriteLine("({0} , {1})", result.ProductID, result.RuleID);
4: }
The output should be similar on this:
( P01 - R_1 )
( P01 - R_2 )
( P01 - R_3 )
( P02 - R_1 )
( P02 - R_2 )
( P02 - R_3)
Conclusion
Cross join operation is useful when performing a Cartesian product
of two sequences object.
However, it can produce very large result sets that may caused a problem
of performance. So use with precautions :)
