Question

Joins can be always expressed as subqueries, and vice versa may not be possible. As joins are symmetric. Can anyone provide me an example for this statement.

As I am unable to get it fully.

Answer 1

I’m not sure the first statement is true anyway.

For example:

SELECT *
FROM x
INNER JOIN y
    ON 1 = 1;

How does one express the nature of a cross join using x and a subquery based on y?

Or even a triangular join:

SELECT *
FROM x
INNER JOIN y
    ON y.val < x.val;

So the first statement must be restricted to cases where there are at most one rows joining so that you can use a scalar subquery?