Oracle 18c

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CREATE TABLE submissions (submission_date, submission_id, hacker_id, score) AS
SELECT DATE '2016-03-01',  1, 1,  80 FROM DUAL UNION ALL
SELECT DATE '2016-03-01',  2, 1,  90 FROM DUAL UNION ALL
SELECT DATE '2016-03-01',  3, 1, 100 FROM DUAL UNION ALL
SELECT DATE '2016-03-01',  4, 2,  90 FROM DUAL UNION ALL
SELECT DATE '2016-03-01',  5, 2, 100 FROM DUAL UNION ALL
SELECT DATE '2016-03-02',  6, 1, 100 FROM DUAL UNION ALL
SELECT DATE '2016-03-02',  7, 2,  90 FROM DUAL UNION ALL
SELECT DATE '2016-03-02',  8, 2, 100 FROM DUAL UNION ALL
SELECT DATE '2016-03-02',  9, 3,  80 FROM DUAL UNION ALL
SELECT DATE '2016-03-02', 10, 3, 100 FROM DUAL;
10 rows affected

CREATE TABLE hackers (hacker_id, name) AS
SELECT 1, 'Alice' FROM DUAL UNION ALL
SELECT 2, 'Betty' FROM DUAL UNION ALL
SELECT 3, 'Carol' FROM DUAL;
3 rows affected

ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD HH24:MI:SS';
WITH calendar (day) AS (
  SELECT DATE '2016-03-01' + LEVEL - 1 AS day
  FROM   DUAL
  CONNECT BY LEVEL <= 15
),
daily_hacker_submissions (submission_date, hacker_id, num_submissions) AS (
  SELECT c.day,
         hacker_id,
         COUNT(*) AS num_submissions
  FROM   calendar c
         LEFT OUTER JOIN submissions s
         ON (
                -- Don't assume dates are always midnight.
                c.day <= s.submission_date
            AND s.submission_date < c.day + 1
            )
  GROUP BY
         c.day,
         s.hacker_id
),
daily_submissions (submission_date, num_hackers, hacker_id ) AS (
  SELECT submission_date,
         COUNT(DISTINCT hacker_id),
         MIN(hacker_id) KEEP (DENSE_RANK LAST ORDER BY num_submissions)
  FROM   daily_hacker_submissions
  GROUP BY
         submission_date
)
SELECT d.submission_date,
       d.num_hackers,
       d.hacker_id,
       h.name
FROM   daily_submissions d
       LEFT OUTER JOIN hackers h
       ON (d.hacker_id = h.hacker_id)


SUBMISSION_DATE
NUM_HACKERS
HACKER_ID
NAME


2016-03-01 00:00:00
2
1
Alice

2016-03-02 00:00:00
3
2
Betty

2016-03-03 00:00:00
0
null
null

2016-03-04 00:00:00
0
null
null

2016-03-05 00:00:00
0
null
null

2016-03-06 00:00:00
0
null
null

2016-03-07 00:00:00
0
null
null

2016-03-08 00:00:00
0
null
null

2016-03-09 00:00:00
0
null
null

2016-03-10 00:00:00
0
null
null

2016-03-11 00:00:00
0
null
null

2016-03-12 00:00:00
0
null
null

2016-03-13 00:00:00
0
null
null

2016-03-14 00:00:00
0
null
null

2016-03-15 00:00:00
0
null
null

SUBMISSION_DATE	NUM_HACKERS	HACKER_ID	NAME
2016-03-01 00:00:00	2	1	Alice
2016-03-02 00:00:00	3	2	Betty
2016-03-03 00:00:00	0	null	null
2016-03-04 00:00:00	0	null	null
2016-03-05 00:00:00	0	null	null
2016-03-06 00:00:00	0	null	null
2016-03-07 00:00:00	0	null	null
2016-03-08 00:00:00	0	null	null
2016-03-09 00:00:00	0	null	null
2016-03-10 00:00:00	0	null	null
2016-03-11 00:00:00	0	null	null
2016-03-12 00:00:00	0	null	null
2016-03-13 00:00:00	0	null	null
2016-03-14 00:00:00	0	null	null
2016-03-15 00:00:00	0	null	null