select person_id, movie_id, position
from credits
where not((not (position is null)) or position = 17);
Supppose that a and b are 3-valued expressions in SQL (true, false, or null). Can the expression (a and b) is null be rewritten as an equivalent boolean combination of a, b, a is null, and b is null?
In boolean logic, the expressin not(a and b) always has the same value as (not a) or (not b). Is that true in 3-valued logic?
What about the expression not(a or b) and the expression (not a) and (not b)?
Is the expression a or (not a) always true in 3-valued logic? If not, can you extend this axiom to make it always true?
Discuss the practical exercises for graph databases. Do you understand the queries and what they are doing?
Our SQL tutorial presented several examples of LEFT JOIN such as