Question:

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 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

My query so far is following:

SELECT DISTINCT
  @s := submission_date,
  COUNT(DISTINCT hacker_id),
  @h := (SELECT hacker_id
         FROM (SELECT DISTINCT
                 hacker_id,
                 @ch := COUNT(hacker_id)
               FROM Submissions
               WHERE submission_date=@s
               ORDER BY @ch
               GROUP BY hacker_id DESC) x
         LIMIT 1),
  @n:=(SELECT name FROM Hackers WHERE hacker_id=@h)
FROM
  Submissions
WHERE 1;

Well this query fails 'SQL sytax error'{MYSQL}, however without GROUP BY this query is rejected as only-full-group-by mode is activated.

Error Message is:

ERROR 1064 (42000) at line 5: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'GROUP BY hacker_id DESC) x LIMIT 1),@n:=(SELECT name FROM Hackers WHERE hacker_i' at line 1

CodePudding user response：

You didn't mention the column name after WHERE.

Try this:

SELECT <your query> FROM Submissions WHERE <column name> = 1;

I think this will be helpful for you.

CodePudding user response：

Though this does not provide correct answer but changing query as suggested by Mr Daniyal solved Syntax issue. Following is final query:

SELECT DISTINCT
  @s := submission_date,
  COUNT(DISTINCT hacker_id),
  @h := (SELECT hacker_id
         FROM Submissions 
         WHERE submission_date=@s 
         GROUP BY hacker_id 
         ORDER BY COUNT(hacker_id) 
         DESC LIMIT 1),
  @n:=(SELECT name FROM Hackers WHERE hacker_id=@h)
FROM
  Submissions
WHERE 1 GROUP BY submission_date;