NULL

DROP TABLE IF EXISTS sales;
CREATE TABLE sales (city TEXT, units INTEGER, note TEXT);
INSERT INTO sales VALUES
  ('Nairobi', 12, 'busy'),
  ('Mombasa', 8, NULL),
  ('Kisumu', 5, NULL);
SELECT city, note FROM sales WHERE note IS NULL;

DROP TABLE IF EXISTS sales;
CREATE TABLE sales (city TEXT, units INTEGER, note TEXT);
INSERT INTO sales VALUES
  ('Nairobi', 12, 'busy'),
  ('Kisumu', 5, NULL);
SELECT city, COALESCE(note, 'none') AS note FROM sales;

DROP TABLE IF EXISTS sales;
CREATE TABLE sales (city TEXT, units INTEGER);
INSERT INTO sales VALUES ('Eldoret', NULL);
SELECT city, units FROM sales WHERE units IS NULL;

DROP TABLE IF EXISTS sales;
CREATE TABLE sales (city TEXT, units INTEGER);
INSERT INTO sales VALUES ('Nakuru', 4), ('Eldoret', NULL);
SELECT city, COALESCE(units, 0) AS units FROM sales;