A cross join:
Returns all possible combinations of rows of two tables. The result contains all rows from the first table; each row from the first table is combined with all rows from the second table.
Doesn’t use a join condition. To create a cross join, omit the ON clause if you’re using JOIN syntax, or omit the WHERE clause if you’re using WHERE syntax.
Seldom is used in practice, as the result is hard to interpret.
Can produce a huge result, even with small tables. If one table contains m rows and the other contains n rows, the result contains m × n rows.
Is a computationally expensive and time-consuming query.
Also is called a Cartesian product or cross product.
SELECT columns ...