Postgresql Update Based on count, min and group by - postgresql

Question

Thank you for taking the time to look at my question.
I've seen similar questions, but not the same depth. Please help!
I would like to update a column all rows in a table that holds user_id and date_created with the lowest date_created for the user_id.
The following select gives me all the rows I would like to update:
select user_id, min(date_created) from mytable s1 where
(select count(1) from mytable s2 where
s1.user_id = s2.user_id group by s2.user_id)
> 1 group by user_id order by user_id;
I would have expected this update to work:
update mytable set join_status = 1 where date_created =
(select min(date_created) from mytable s1 where
(select count(1) from simplepay_payment s2 where
s1.user_id = s2.user_id group by s2.user_id)
> 1 group by user_id);
But is gave the following error:
ERROR: more than one row returned by a subquery used as an expression
I've tried a few different solutions, but nothing seems to help.
Does anyone have any ideas fro me?
Thanks again.

Answer 1

Change your SQL to:
update mytable set join_status = 1 where date_created IN
(select min(date_created) from mytable s1 where
(select count(1) from simplepay_payment s2 where
s1.user_id = s2.user_id group by s2.user_id)
> 1 group by user_id);
Read more on row comparison in the docs.
EDIT:
In the subquery you're performing GROUP BY user_id. This means that you will receive many rows, based on the number of unique user_id values in your simplepay_payment table.
To make your query working as expected, you should join using 2 columns: user_id and date_created. As you've mentioned, you already have the query that gives you the correct results, so you can use it like this:
WITH desired AS (
SELECT user_id, min(date_created) AS mindt
FROM mytable s1 where
(SELECT count(1) FROM mytable s2
WHERE s1.user_id = s2.user_id GROUP BY s2.user_id) > 1
GROUP BY user_id)
UPDATE mytable m SET join_status = 1 FROM desired d
WHERE d.user_id = m.user_id AND d.mindt = m.date_created;
I've wrapped in your query into the Common Table Expression and used it in the UPDATE statement. You can add RETURNING m.* at the end of the query to see the rows that had been updated and their new values.
You can test this query on SQL Fiddle.
EDIT2:
Common Table Expressions (WITH-queries) are not available before version 9.1 for UPDATE statements. You can simply move the CTE subquery into the update, like this:
UPDATE mytable m SET join_status = 1 FROM (
SELECT user_id, min(date_created) AS mindt
FROM mytable s1 where
(SELECT count(1) FROM mytable s2
WHERE s1.user_id = s2.user_id GROUP BY s2.user_id) > 1
GROUP BY user_id) d
WHERE d.user_id = m.user_id AND d.mindt = m.date_created;