sql 代码显示每个类别的最大值，该类别中有多个最大值？

Question

i have table => post

     content || category || views
   -------------------------------
     text 1  |  cat2     | 1000
     text 2  |  cat2     | 1000
     text 3  |  cat1     | 500
     text 4  |  cat1     | 400

i want to display max value from each category but it is showing as:

     content || category || views
   -------------------------------
     text 1  |  cat2     | 1000
     text 2  |  cat2     | 1000
     text 3  |  cat1     | 500

my desire output:

     content || category || views
   -------------------------------
     text 1  |  cat2     | 1000
     text 3  |  cat1     | 500

Basically i want to display only one max value from each category even in case where it has more than one max value.In case if there is more than on max values then i want it to show that value which is first in ordering by content column coln ie text 1.

query i run:

SELECT t1.*
FROM post t1
INNER JOIN
(
    SELECT category, MAX(views) AS max_views
    FROM post
    GROUP BY category
) t2
    ON t1.category = t2.category AND t1.views = t2.max_views order by views DESC

Answer 1

There are various ways to do this. If you have a unique id for each row, then you can do:

SELECT p.*
FROM post p
WHERE p.id = (SELECT p2.id
              FROM post p2
              WHERE p2.category = p.category
              ORDER BY p2.views DESC
              LIMIT 1
             );

It is preferable to do this with a primary key, if you have one. The above query should be quite fast if you have an index on post(category, views) .

You can use content if it is unique:

SELECT p.*
FROM post p
WHERE p.content = (SELECT p2.content
                   FROM post p2
                   WHERE p2.category = p.category
                   ORDER BY p2.views DESC
                   LIMIT 1
                  );

Otherwise, some sort of multiple aggregation comes into play, such as:

SELECT p.category, MAX(p.content) as content, MAX(p.views) as max_views
FROM post p INNER JOIN
     (SELECT category, MAX(views) AS max_views
      FROM post
      GROUP BY category
     ) p2
    ON p.category = p2.category AND p.views = p2.max_views GROUP BY p.category;

Of course, MySQL 8+ solves this by implementing the ANSI-standard ROW_NUMBER() functionality.

I note that your queries use the generic t as a table alias. That is a bad idea. You should be using an abbreviation for the table name. It makes the queries much easier to follow.

Answer 2

You can get the most viewed content in each category by selecting them in a subquery, along with the max view count.

select distinct t1.category,
    (select t2.content
    from post t2
    where t2.category = t1.category
    order by t2.views
    limit 1
    ) best_content,
    max(t1.views) views
from post t1
group by t1.category

I hope it helps

Answer 3

A simple solution can be the Use of max function with group by as follows:

SELECT content ,  category, max(views)  FROM post WHERE GROUP by category ;