Cross Joins - cross-product of two data sources, aka a Cartesian product. Each row from a data source is matched with ALL rows in the other data source. (DataSource1 * DataSource2)

cross_join.sql

--how many rows?
SELECT COUNT(*)
FROM HumanResources.Employee;

--what gets returned?
SELECT e.BusinessEntityId, edh.DepartmentId
FROM HumanResources.Employee AS e
CROSS JOIN HumanREsources.EmployeeDepartmentHistory AS edh;

--practical use?
SELECT TOP 100000
  ROW_NUMBER() OVER (ORDER BY sv1.number) AS num
FROM [master].[dbo].[spt_values] AS sv1
CROSS JOIN [master].[dbo].[spt_values] AS sv2;