Sqlite / populate new column that ranks the existing rows - sqlite

Question

I've a SQLite database table with the following columns:
| day | place | visitors |
-------------------------------------
| 2021-05-01 | AAA | 20 |
| 2021-05-01 | BBB | 10 |
| 2021-05-01 | CCC | 3 |
| 2021-05-02 | AAA | 5 |
| 2021-05-02 | BBB | 7 |
| 2021-05-02 | CCC | 2 |
Now I would like to introduce a column 'rank' which indicates the rank according to the visitors each day. Expected table would look like:
| day | place | visitors | Rank |
------------------------------------------
| 2021-05-01 | AAA | 20 | 1 |
| 2021-05-01 | BBB | 10 | 2 |
| 2021-05-01 | CCC | 3 | 3 |
| 2021-05-02 | AAA | 5 | 2 |
| 2021-05-02 | BBB | 7 | 1 |
| 2021-05-02 | CCC | 2 | 3 |
Populating the data for the new column Rank can be done with a program like (Pseudocode).
for each i_day in all_days:
SELECT
ROW_NUMBER () OVER (ORDER BY `visitors` DESC) Day_Rank, place
FROM mytable
WHERE `day` = 'i_day'
for each i_place in all_places:
UPDATE mytable
SET rank= Day_Rank
WHERE `Day`='i_day'
AND place = 'i_place'
Since this line by line update is quite inefficient, I'm searching how to optimize this with a SQL sub query in combination with the UPDATE.
(does not work so far...)
for each i_day in all_days:
UPDATE mytable
SET rank= (
SELECT
ROW_NUMBER () OVER (ORDER BY `visitors` DESC) Day_Rank
FROM mytable
WHERE `day` = 'i_day'
)

Answer 1

Typically, this can be done with a subquery that counts the number of rows with visitors greater than the value of visitors of the current row:
UPDATE mytable
SET Day_Rank = (
SELECT COUNT(*) + 1
FROM mytable m
WHERE m.day = mytable.day AND m.visitors > mytable.visitors
);
Note that the result is actually what RANK() would return, if there are ties in the values of visitors.
See the demo.
Or, you could calculate the rankings with ROW_NUMBER() in a CTE and use it in a subquery:
WITH cte AS (
SELECT *, ROW_NUMBER() OVER (PARTITION BY day ORDER BY visitors DESC) rn
FROM mytable
)
UPDATE mytable
SET Day_Rank = (SELECT rn FROM cte c WHERE (c.day, c.place) = (mytable.day, mytable.place));
See the demo.
Or, if your versipn of SQLite is 3.33.0+ you can use the join-like UPDATE...FROM... syntax:
UPDATE mytable AS m
SET Day_Rank = t.rn
FROM (
SELECT *, ROW_NUMBER() OVER (PARTITION BY day ORDER BY visitors DESC) rn
FROM mytable
) t
WHERE (t.day, t.place) = (m.day, m.place);