Cross join error with unnest array and user supplied data

Question

I am trying to create a cross join in postgres

SELECT * FROM (SELECT unnest(ARRAY[1,2])) AS t1(product_id)
CROSS JOIN
SELECT * FROM (SELECT unnest(ARRAY[5])) AS t2(category_id);

This gives error. I can not figure the problem.

Answer 1

Your query has unnecessary levels of nesting, which finally cause problem since some of the derived tables (ie subqueries) are not aliases.

You could just phrase this as:

select t1.product_id, t2.category_id
from unnest(array[1,2]) as t1(product_id)
cross join unnest(array[5]) AS t2(category_id);

Answer 2

This works as well

select * from 
    unnest(array[1,2]) t1(product_id), 
    unnest(array[5]) t2(category_id);