mysql中出现频率最高的值

Question

I am stuck on a problem where I need to find the count of the most frequent value.

With this I will get the Table with all the different cars, their problem count and the most frequent problem.

Now I want to count how many times does the most frequent problem occur. I think I need to do something with subqueries but I can't seem to figure it out.

Can anyone help me out or lead me to the right direction please?

Answer 1

Did I get you right, that you would like to count the occurence of the most frequent ProblemCount?

If so: This is not the cleanest solution, but I hope, this will help you.

SELECT
  ProblemCount,
  COUNT(ProblemCount)
FROM(
  SELECT Cars.Model, 
  COUNT(Jobs.JobType) AS ProblemCount, 
  Jobs.JobType AS MostCommonProblemType
  FROM Cars
  INNER JOIN Jobs ON Jobs.CarId = Cars.Id 
  GROUP BY Cars.Model
) A
WHERE ProblemCount = (SELECT MAX(B.ProblemCount) FROM (SELECT 
  COUNT(Jobs.JobType) AS ProblemCount
  FROM Cars
  INNER JOIN Jobs ON Jobs.CarId = Cars.Id 
  GROUP BY Cars.Model) B)
GROUP BY ProblemCount;

Answer 2

The best solution is to use a window function, rank() :

SELECT *
FROM (SELECT c.Model, COUNT(*) AS ProblemCount, 
             j.JobType AS MostCommonProblemType,
             RANK) OVER (PARTITION BY c.Model ORDER BY COUNT(*) DESC) as ranking
      FROM Cars c INNER JOIN
           Jobs j
           ON j.CarId = c.Id 
      GROUP BY c.Model, j.JobType
     ) m
WHERE ranking = 1;

Here is a db<>fiddle.

Note that use a table aliases; this makes the query easier to write and to read. Also, you can use COUNT(*) . There seems to be no need to check if Job(JobType) is not NULL , which is what COUNT(j.JobType) does.

Answer 3

I used, as you, that strange feature of mysql, using a column that is not grouped and not aggregated, and made the needed subselect:

SELECT c.Model, 
COUNT(j.JobType) AS ProblemCount,
j.JobType as MostCommonProblemType
, (select count(*) from Jobs where jobs.Jobtype = j.JobType 
   and CarId in (select Id from Cars c2 where c2.model = c.model) 
  ) as ProblemCount
FROM Cars c
INNER JOIN Jobs j ON j.CarId = c.Id 
GROUP BY c.Model
ORDER BY COUNT(j.JobType) DESC;

Output

Model	ProblemCount	MostCommonProblemType	ProblemCount
Ford Mustang	8	Engine	4
Ford Focus	6	Engine	3
Ford Puma	2	Maintenance	2

fiddle