A WHERE clause keeps rows whose condition is true. A condition that evaluates to NULL does not pass. That is why amount = NULL cannot find missing amounts: use IS NULL. For a complete reporting window, write the start as inclusive and the next boundary as exclusive. An end-of-day timestamp can have fractions of a second, so a hard-coded 23:59:59 boundary can drop valid records.
Aster Supply uses statuses to distinguish fulfilled orders, canceled orders and refunds. Read the task's status rule instead of assuming that every positive amount belongs in revenue. Combine conditions with AND and group alternatives with parentheses. ORDER BY controls presentation, while WHERE controls which records qualify. A LIMIT without an order doesn't promise which rows you'll see.