Question

The following is an example structure:

table `pligg`
#id   #alpha     #num
1       a        null
2       b        null
3       c        null
4       a        null
5       d        null
6       b        null
7       a        null
8       e        null

I’d like to update the databse like this after a single mysqli query:

table `pligg`
#id   #alpha     #num
1       a        1
2       b        1
3       c        1
4       a        2
5       d        1
6       b        2
7       a        3
8       e        1

What I’m trying to do is to update the column num with the number of duplicate.
I tried this query, but in vain

UPDATE pligg SET 'num' = COUNT(DISTINCT alpha) WHERE 'id'<id

Answer 1

This should do the trick:

UPDATE pligg a
INNER JOIN
(
    SELECT a.id, a.alpha, COUNT(1) AS dup_cnt
    FROM pligg a
    INNER JOIN pligg b ON a.id >= b.id AND a.alpha = b.alpha
    GROUP BY a.id, a.alpha
) b ON a.id = b.id
SET a.num = b.dup_cnt

---

SQLFiddle Demo