Relational Algebra

Question

what is difference between full outer join and cross product?

Comments

A cross join produces a cartesian product between the two tables, returning all possible combinations of all rows. It has no on clause because you're just joining everything to everything,

A full outer join is a combination of a left outer and right outer join. It returns all rows in both tables that match the query's where clause, and in cases where the on condition can't be satisfied for those rows it puts null values in for the unpopulated fields

for further reference https://stackoverflow.com/questions/3228871/sql-server-what-is-the-difference-between-cross-join-and-full-outer-join

Answer 1

No: the Cartesian product is not the same as SQL FULL OUTER JOIN.

For example: if A = {1,2} and B = ∅, then A × B = {1,2} × ∅ = ∅, i.e. the Cartesian product yield the empty set. But, since FULL OUTER JOIN does not require each record in the two joined tables to have a matching record, if B is empty and A is not, FULL OUTER JOIN will still return some rows.

However, CROSS JOIN will return the Cartesian product of rows from tables in the join.

Source: https://math.stackexchange.com/questions/668885/is-cartesian-product-same-as-sql-full-outer-join