MySQL: Limiting number of results received based on a column value | Combining queries

问题

I've done research on this problem, but am having trouble finding a solution.

I have the following query that gives me a list of "some_id"s:

SELECT some_id FROM example GROUP BY some_id

And I have the following query that will get a list of the 5 most recent entries for a row that has "some_id" equal to a number.

SELECT * FROM example
WHERE some_id = 1
ORDER BY last_modified DESC
LIMIT 5

How can I get the the top 5 most recent entries from the table "example" for each "some_id", using only one query? If there are less than 5 entries for a "some_id", it is okay to include them, if that makes things less complex.

Many thanks!

回答1:

Found the answer when looking at the first answer in the following post:

How do I limit the number of rows per field value in SQL?

I've modified it to fit my specific needs:

SELECT * FROM
(
    SELECT *, @num := if(@some_id = some_id, @num := @num + 1, 1) as row_num,
           @some_id := some_id as some_id
    FROM example
    ORDER BY last_modified DESC
) as e
WHERE row_num <= 5

回答2:

Hi Please use Group by and having clause for check limit for every id..

SELECT * FROM `example`
GROUP BY some_id
HAVING count( * ) <= 5
ORDER BY last_modified DESC

It will check for every some_id and check if number of rows for some_id is <=5 then it will be display result.

来源：https://stackoverflow.com/questions/10353436/mysql-limiting-number-of-results-received-based-on-a-column-value-combining-q