In the exam from 2016, moed A, question 1, part b, subsection iii, we are asked to write a query that retrieves all the chefs that won at least once and haven't lost once in a show with a theme that is their speciality without using INTERSECT, EXCEPT, EXISTS and IN using only R/L JOIN and a FULL OUTER JOIN.
Whilel I do understand the usage of a LEFT JOIN in the solution the ON condition seems to be contradictory:
A.id = B.id AND B.id = NULL
This seems weird because if both conditions hold it means that both id's are NULL and because we know that there are no NULL values in the table it means that nothing will be returned.
Perhapbs the condition should be A.id <> B.id OR B.id = NULL?