A (vague, handwavy) use case would be if you had a table of Towns, and a table of Pizza_Chains; by iterating over the joined table, you would see every permutation of Towns and Pizza_Chains, and (by inputting address info) you could produce a new table giving the address for each Pizza_Chain in each Town.
The cross-join would be used once, to create the new table. You want to use cross joins as little as possible, so a single-use cross join is reasonable. I admit it's a pretty stretched example; I suspect a better example might be found in a more abstract field, like maths, but I'm not feeling abstract enough right now to work one out.
You make a cross join like this:
SELECT Towns.id, Towns.name, Pizza_Chains.id, Pizza_Chains.name
FROM Towns, Pizza_Chains
I.e. you simply don't have a WHERE clause.
I don't think I've ever had occasion to use a cross join. It's not a very useful query, because the resultset doesn't give you any information - or rather, it tells you every possible result you could get, if you tried the query with every possible WHERE clause.
A (vague, handwavy) use case would be if you had a table of Towns, and a table of Pizza_Chains; by iterating over the joined table, you would see every permutation of Towns and Pizza_Chains, and (by inputting address info) you could produce a new table giving the address for each Pizza_Chain in each Town.
The cross-join would be used once, to create the new table. You want to use cross joins as little as possible, so a single-use cross join is reasonable. I admit it's a pretty stretched example; I suspect a better example might be found in a more abstract field, like maths, but I'm not feeling abstract enough right now to work one out.
You make a cross join like this:
SELECT Towns.id, Towns.name, Pizza_Chains.id, Pizza_Chains.name FROM Towns, Pizza_Chains
I.e. you simply don't have a WHERE clause.
I don't think I've ever had occasion to use a cross join. It's not a very useful query, because the resultset doesn't give you any information - or rather, it tells you every possible result you could get, if you tried the query with every possible WHERE clause.