MySQL update value from the same table with count - mysql

Question

What I want to do is to set every patient its unique patient code which starts with 1 and it's not based on row id. Id only specifies order. Something like this:
patient_id patient_code
2 1
3 2
4 3
This is my query:
UPDATE patients p1
SET p1.patient_code = (
SELECT COUNT( * )
FROM patients p2
WHERE p2.patient_id <= p1.patient_id
)
But it is throwing error:
#1093 - You can't specify target table 'p1' for update in FROM clause
I found this thread: Mysql error 1093 - Can't specify target table for update in FROM clause.But I don't know how to apply approved answer this to work with subquery WHERE which is necessary for COUNT.

Answer 1

UPDATE
patients AS p
JOIN
( SELECT
p1.patient_id
, COUNT(*) AS cnt
FROM
patients AS p1
JOIN
patients AS p2
ON p2.patient_id <= p1.patient_id
GROUP BY
p1.patient_id
) AS g
ON g.patient_id = p.patient_id
SET
p.patient_code = g.cnt ;

Answer 2

I found working solution, but this is just workaround:
SET #code=0;
UPDATE patients SET patient_code = (SELECT #code:=#code+1 AS code)

Answer 3

Try this,
UPDATE patients p1 INNER JOIN
(
SELECT COUNT(*) as count,patient_id
FROM patients
group by patient_id
)p2
SET p1.patient_code=p2.count
WHERE p2.patient_id <= p1.patient_id
SQL_LIVE_DEMO

Answer 4

Thanks to Mari's answer I found a solution to my similar problem. But I wanted to add a bit of an explanation which for me at first wasn't too clear from his answer.
What I wanted to do would have been as simple as the following:
UPDATE my_comments AS c
SET c.comment_responses = (
SELECT COUNT(c1.*) FROM my_comments AS c1
WHERE c.uid = c.parent_uid
);
Thanks to Mari I then found the solution on how to achieve this without running into the error You can't specify target table 'p1' for update in FROM clause:
UPDATE my_comments AS c
INNER JOIN (
SELECT c1.parent_uid, COUNT(*) AS cnt
FROM my_comments AS c1
WHERE c1.parent_uid <> 0
GROUP BY c1.parent_uid
) AS c2
SET c.comment_responses = c2.cnt
WHERE c2.parent_uid = c.uid;
My problems before getting to this solution were 2:
the parent_uid field doesn't always contain an id of a parent which is why I added the WHERE statement in the inner join
I didn't quite understand why I would need the GROUP BY until I executed the SELECT statement on it's own and the answer is: because COUNT groups the result and really counts everything. In order to prevent this behavior the GROUP BY is needed. In my case I didn't have to group it by uid though but the parent_uid to get the correct count. If I grouped it by uid the COUNT would always be 1 but the parent_uid existed multiple times in the result. I suggest you check the SELECT statement on it's own to check if it's the result you expect before you execute the full UPDATE statement.