How to update a PostgreSQL table with a count of duplicate items - sql

Question

I found two bugs in a program that created a lot of duplicate values:
an 'index' was created instead of a 'unique index'
a duplication checks wasn't integrated in one of 4 twisted routines
So I need to go in and clean up my database.
Step one is to decorate the table with a count of all the duplicate values (next I'll look into finding the first value, and then migrating everything over )
The code below works, I just recall doing a similar "update from select count" on the same table years ago, and I did it in half as much code.
Is there a better way to write this?
UPDATE
shared_link
SET
is_duplicate_of_count = subquery.is_duplicate_of_count
FROM
(
SELECT
count(url) AS is_duplicate_of_count
, url
FROM
shared_link
WHERE
shared_link.url = url
GROUP BY
url
) AS subquery
WHERE
shared_link.url = subquery.url
;

Answer 1

You query is fine, generally, except for the pointless (but also harmless) WHERE clause in the subquery:
UPDATE shared_link
SET is_duplicate_of_count = subquery.is_duplicate_of_count
FROM (
SELECT url
, count(url) AS is_duplicate_of_count
FROM shared_link
-- WHERE shared_link.url = url
GROUP BY url
) AS subquery
WHERE shared_link.url = subquery.url;
The commented clause is the same as
WHERE shared_link.url = shared_link.url
and therefore only eliminating NULL values (because NULL = NULL is not TRUE), which is most probably neither intended nor needed in your setup.
Other than that you can only shorten your code further with aliases and shorter names:
UPDATE shared_link s
SET ct = u.ct
FROM (
SELECT url, count(url) AS ct
FROM shared_link
GROUP BY 1
) AS u
WHERE s.url = u.url;
In PostgreSQL 9.1 or later you might be able to do the whole operation (identify dupes, consolidate data, remove dupes) in one SQL statement with aggregate and window functions and data-modifying CTEs - thereby eliminating the need for an additional column to begin with.