SQL Group By with an Order By

In all versions of MySQL, simply alias the aggregate in the SELECT list, and order by the alias:

SELECT COUNT(id) AS theCount, `Tag` from `images-tags`
GROUP BY `Tag`
ORDER BY theCount DESC
LIMIT 20

MySQL prior to version 5 did not allow aggregate functions in ORDER BY clauses.

You can get around this limit with the deprecated syntax:

SELECT COUNT(id), `Tag` from `images-tags`
GROUP BY `Tag`
ORDER BY 1 DESC
LIMIT 20

1, since it's the first column you want to group on.

I don't know about MySQL, but in MS SQL, you can use the column index in the order by clause. I've done this before when doing counts with group bys as it tends to be easier to work with.

So

SELECT COUNT(id), `Tag` from `images-tags`
GROUP BY `Tag`
ORDER BY COUNT(id) DESC
LIMIT 20

Becomes

SELECT COUNT(id), `Tag` from `images-tags`
GROUP BY `Tag`
ORDER 1 DESC
LIMIT 20