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15_days_learning_SQL.sql
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15_days_learning_SQL.sql
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/*
15 days of learning SQL (advanced JOIN)
Julia conducted a 15 days of learning SQL contest. The start date of the contest was *March 01, 2016* and the end date was *March 15, 2016*.
Write a query to print total number of unique hackers who made at least 1 submission each day (starting on the first day of the contest), and
find the *hacker_id* and *name* of the hacker who made maximum number of submissions each day.
If more than one such hacker has a maximum number of submissions, print the lowest *hacker_id*.
The query should print this information for each day of the contest, sorted by the date.
- Input Format
The following tables hold contest data:
- *Hackers:* The *hacker_id* is the id of the hacker, and *name* is the name of the hacker.
- *Submissions:* The *submission_date* is the date of the submission, *submission_id* is the id of the submission,
*hacker_id* is the id of the hacker who made the submission, and *score* is the score of the submission.
- Sample Input**
For the following sample input, assume that the end date of the contest was *March 06, 2016*.
*Hackers* Table:
*Submissions* Table:
- Sample Output
2016-03-01 4 20703 Angela
2016-03-02 2 79722 Michael
2016-03-03 2 20703 Angela
2016-03-04 2 20703 Angela
2016-03-05 1 36396 Frank
2016-03-06 1 20703 Angela
- Explanation
On *March 01, 2016* hackers 20703, 36396, 53473, and 79722 made submissions. There are 4 unique hackers who made at least one submission each day.
As each hacker made one submission, 20703 is considered to be the hacker who made maximum number of submissions on this day. The name of the hacker is *Angela*.
On *March 02, 2016* hackers 15758, 20703, and 79722 made submissions. Now 20703 and 79722 were the only ones to submit every day, so there are 2 unique hackers
who made at least one submission each day. 79722 made 2 submissions, and name of the hacker is *Michael*.
On *March 03, 2016* hackers 20703, 36396, and 79722 made submissions. Now 20703 and 79722 were the only ones, so there are 2 unique hackers who made at least
one submission each day. As each hacker made one submission so 20703 is considered to be the hacker who made maximum number of submissions on this day. The name of the hacker is *Angela*.
On *March 04, 2016* hackers 20703, 44065, 53473, and 79722 made submissions. Now 20703 and 79722 only submitted each day, so there are 2 unique hackers who made at
least one submission each day. As each hacker made one submission so 20703 is considered to be the hacker who made maximum number of submissions on this day. The name of the hacker is *Angela*.
On *March 05, 2016* hackers 20703, 36396, 38289 and 62529 made submissions. Now 20703 only submitted each day, so there is only 1 unique hacker who made at least one
submission each day. 36396 made 2 submissions and name of the hacker is *Frank*.
On *March 06, 2016* only 20703 made submission, so there is only 1 unique hacker who made at least one submission each day. 20703 made 1 submission and name of the hacker is *Angela*.
*/
WITH ranked_subs AS (
SELECT
submission_date,
hacker_id,
RANK() OVER(
PARTITION BY submission_date
ORDER BY submission_date, COUNT(*) DESC, hacker_id) rnk
FROM
submissions
GROUP BY
submission_date, hacker_id
),
counted_subs AS (
SELECT
submission_date,
COUNT(DISTINCT hacker_id) AS hackers_count
FROM
(SELECT
subs1.*,
DENSE_RANK() OVER(
ORDER BY submission_date) AS date_rank,
DENSE_RANK() OVER(
PARTITION BY hacker_id ORDER BY submission_date) AS hacker_rank
FROM submissions AS subs1) AS counted_subs_tmp
WHERE date_rank = hacker_rank
GROUP BY submission_date)
SELECT
cs.submission_date,
cs.hackers_count,
rs.hacker_id,
h.name
FROM
counted_subs AS cs
JOIN
ranked_subs AS rs
ON
cs.submission_date = rs.submission_date
AND rs.rnk = 1
JOIN
hackers AS h
ON
rs.hacker_id = h.hacker_id
;