NULL means "no value": unknown, not applicable, not entered. It is not zero and not an empty string, and it compares equal to nothing, not even itself: city = NULL is never true. The test is city IS NULL and its opposite city IS NOT NULL.
NULLs also drop out of arithmetic (price + NULL is NULL) and are skipped by aggregate functions such as AVG, which is usually what you want and occasionally a trap. One customer in the practice data has no city.
SELECT name FROM employees WHERE manager_id IS NULL;Your task
Show the name of the customers whose city is not recorded.
Hint 1
city IS NULL, not city = NULL.
Show the solution
SELECT name FROM customers WHERE city IS NULL;