Question

How is the best way to do a ‘running total’ system like the tags search on stackoverflow? If I click on ‘php’ for example, its show the total amount of items on ‘each’ other tags, and its very fast. How can I do this in php with mysql?

Answer 1

It’s a query that “looks” like that

SELECT T2.Tag, COUNT(*)
FROM SO_Posts P1
JOIN Post_Tags T1 ON P.PostId = T1.PostId
JOIN Post_Tags T2 ON P.PostId = T2.PostId
GROUP BY T2.Tag
WHERE T1.Tag = 'PHP'
ORDER BY COUNT(*) DESC

This query makes the plausible assumption that the Posts (Questions) on SO are stored in two tables;
SO_Posts, containing one record per Post, and holding info such as a PostId (Primary Key), the question itself, the date, the title etc.
and
Post_Tags which associates a given Post (by its Post_Id) with a Tag (or more likely a TagId since tags ought to be normalized, but that’s a detail). For a given Post, there are as many records in Post_Tags as there are different tags attached to the post.
Note: in effect the structure of the SO Posts database is more complicated, with various tables for storing comments, replies etc. but with regards to the Post-to-Tag relationship, this two-table layout (or more likely a 3 tables layout allowing to have a tagId in the Post_Tags rather than the tag itself) captures the essence of how it is possible, easy and fast (provided the right indexes) to show these filtered agregate counts.

The idea is to find all PostIDs associated with the targeted tag (here ‘PHP’) (looked-up in “T1”) and then to aggregate all the Posts (in “T2”), by Tag.

Note that the main table SO_Posts is not necessary here, but it would likely be part of the query, for example to allow adding extra criteria such as say the Post status (not being closed…).