Many SQL books and tutorials recommend that you “avoid cross joins”
or “beware of Cartesian products” when writing your SELECT statements,
which occur when you don't express joins between your tables. It’s true
that you need to ensure that your join conditions are adequately stated
so that you don’t accidentally produce this effect, but it is not true
that you should avoid these types of joins in every situation.
Cross Joins produce results that consist of every combination of rows from two or more tables. That means if table A has 3 rows and table B has 2 rows, a CROSS JOIN will result in 6 rows. There is no relationship established between the two tables – you literally just produce every possible combination.
The danger here, of course, is that if you have table A with 10,000 rows and Table B with 30,000 rows, and you accidentally create the product of these two tables, you will end up with a 300,000,000 row result -- probably not a good idea. (Though it is great for creating test data and the like.)
Cross Joins produce results that consist of every combination of rows from two or more tables. That means if table A has 3 rows and table B has 2 rows, a CROSS JOIN will result in 6 rows. There is no relationship established between the two tables – you literally just produce every possible combination.
The danger here, of course, is that if you have table A with 10,000 rows and Table B with 30,000 rows, and you accidentally create the product of these two tables, you will end up with a 300,000,000 row result -- probably not a good idea. (Though it is great for creating test data and the like.)
