How to return rows listed in descending order of COUNT(*)?

Question

I have a table called foo with these fields:

- id

- type

- parentId

I want to select a list of parent IDS, in the descending

Answer 1

Simply by applying a GROUP BY clause, and assuming you have an index , FOREIGN KEY, or PRIMARY KEY on parentId, the performance should be quite good. (parentId looks like it is likely a FORIEGN KEY, so be sure to define the constraint to enforce indexing).

SELECT `parentId`
FROM `foo`
GROUP BY `parentId`
ORDER BY COUNT(*) DESC