Postgres - updates with join gives wrong results - postgresql

Question

I'm having some hard time understanding what I'm doing wrong.
The result of this query shows the same results for each row instead of being updated by the right result.
My DATA
I'm trying to update a table of stats over a set of business
business_stats ( id SERIAL,
pk integer not null,
b_total integer,
PRIMARY KEY(pk)
);
the details of each business are stored here
business_details (id SERIAL,
category CHARACTER VARYING,
feature_a CHARACTER VARYING,
feature_b CHARACTER VARYING,
feature_c CHARACTER VARYING
);
and here a table that associate the pk with the category
datasets (id SERIAL,
pk integer not null,
category CHARACTER VARYING;
PRIMARY KEY(pk)
);
WHAT I DID (wrong)
UPDATE business_stats
SET b_total = agg.total
FROM business_stats b,
( SELECT d.pk, count(bd.id) total
FROM business_details AS bd
INNER JOIN datasets AS d
ON bd.category = d.category
GROUP BY d.pk
) agg
WHERE b.pk = agg.pk;
The result of this query is
| id | pk | b_total |
+----+----+-----------+
| 1 | 14 | 273611 |
| 2 | 15 | 273611 |
| 3 | 16 | 273611 |
| 4 | 17 | 273611 |
but if I run just the SELECT the results of each pk are completely different
| pk | agg.total |
+----+-------------+
| 14 | 273611 |
| 15 | 407802 |
| 16 | 179996 |
| 17 | 815580 |
THE QUESTION
why is this happening?
why is the WHERE clause not working?
Before writing this question I've used as reference these posts: a, b, c

Answer 1

Do the following (I always recommend against joins in Updates)
UPDATE business_stats bs
SET b_total =
( SELECT count(c.id) total
FROM business_details AS bd
INNER JOIN datasets AS d
ON bd.category = d.category
where d.pk=bs.pk
)
/*optional*/
where exists (SELECT *
FROM business_details AS bd
INNER JOIN datasets AS d
ON bd.category = d.category
where d.pk=bs.pk)

Answer 2

The issue is your FROM clause. The repeated reference to business_stats means you aren't restricting the join like you expect to. You're joining agg against the second unrelated mention of business_stats rather than the row you want to update.
Something like this is what you are after (warning not tested):
UPDATE business_stats AS b
SET b_total = agg.total
FROM
(...) agg
WHERE b.pk = agg.pk;