MySQL update value from the same table with count

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

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