7-day moving average of daily sales

Table daily_sales:

Days without sales have no row. Return a 7-day moving average of amount for every calendar day from the first to the last day in the table.

• A missing day counts as 0 sales.
• For each day, average that day and the 6 days before it; at the start of the range use the days that exist (fewer than 7).
• Round to 2 decimals.

Output columns: day, moving_avg. Order by day.

CREATE TABLE daily_sales (
  day date PRIMARY KEY,
  amount int NOT NULL
);