Sometimes you need to show results from table A but also include the count of related records from table B. For example: how many items are in each purchase order, how many comments in each post, how many galaxies in each universe (wait, what?), …
The following query shows one way of selecting the title and id from a table Posts, the count of the related records in a table Comments and then sorts descending by that count. fk_post is the foreign key of the post in the Comments table.
As always, there are multiple ways of doing it. So if you are not going to add more to the subquery, you might prefer to just use a LEFT JOIN with a GROUP BY: