How to check if a column in a query can produce NULL?

I am scheming how to implement a type generator for arbitrary SQL queries.

I found that for any query, the simplest way to get the types of every column is to create a temporary table and then inspect that table. However, this tells nothing about which columns can be NULL.

I am interested to find a solution to identifying which columns can be NULL for any arbitrary query with the goal of generating types for that query.

What’s the way to go about it?