NULL, LIKE, IN, and BETWEEN are operators which can be preceded by NOT. In the case of NULL, the word 'IS' is needed. The condition
WHERE NOT (firstname IS NULL)
can also be written as
WHERE firstname IS NOT NULL.
Find all customers who have a first name, i.e., are not companies:
SELECT firstname, name, city
FROM customer
WHERE firstname IS NOT NULL
Find the customers who are not a company:
SELECT name, city
FROM customer
WHERE title NOT LIKE 'Co%'
Find the customers who do not live in Dallas or New York:
SELECT name, city, state, zip
FROM customer
WHERE city NOT IN ('Dallas','New York')
Find the customers who have either a positive account or a considerable negative account:
SELECT title, name, city, account
FROM customer
WHERE account NOT BETWEEN -10 AND 0
The preceding example can also be formulated differently:
SELECT title, name, city, account
FROM customer
WHERE NOT (account >= -10 AND ACCOUNT <= 0)
or else:
SELECT title, name, city, account
FROM customer
WHERE account < -10 OR account > 0