Your Query Exercise was to find foreign key problems: data that might stop us from successfully implementing foreign keys in the Stack Overflow database.
Question 1: Write queries to find the problematic data.
Montro1981’s thorough answer broke the challenge into two parts: first, check to see if there is ANY bad data, and if so, check to see how pervasive it is. What I really like about this query below is that it recognizes that:
Each table, and each column, might have different foreign key problems Some of those problems might be big, and some might be small Some