Very nice. It's time to review what we've learned about NULLs so far:

• Rule number one: Never trust a NULL. Keep your eyes open.
• Use the operator IS NULL to check if the column is NULL.
• Use the operator IS NOT NULL to check if the column is not NULL.
• The equality and non-equality conditions (price = 7, price = NULL, price > 15) are never true when the argument is NULL.
• Arithmetical operations (e.g. a + b) and most functions (e.g. a || b, length(a)) will return a NULL if any of the values is a NULL.
• COALESCE(x, y, ...) returns the first non-NULL argument.
• NULLIF(x, y) returns x when x != y or NULL when x = y.

Are you ready for a short quiz?