copy (with update) certain content from the same table - php

Question

I wanted to UPDATE the value of my below table (row & col_md) :
Current Data
| id | id_cat | row | col_md |
| --- | ------ | ---- | ------ |
| 1 | 1 | 1 | 4 |
| 2 | 1 | 2 | 5 |
| 3 | 1 | 3 | 5 |
| 4 | 2 | 1 | 3 |
| 5 | 2 | 2 | 4 |
| 6 | 2 | 2 | 4 |
| 7 | 3 | 1 | 12 |
| 8 | 3 | 1 | 12 |
| 9 | 3 | 2 | 3 |
That may look something like the below table. (I want to have the same content of rows that id_cat=1 have, in rows with id_cat=2 & 3).
Required Data:
| id | id_cat | row | col_md |
| --- | ------ | ---- | ------ |
| 1 | 1 | 1 | 4 |
| 2 | 1 | 2 | 5 |
| 3 | 1 | 3 | 5 |
| 4 | 2 | 1 | 4 |
| 5 | 2 | 2 | 5 |
| 6 | 2 | 3 | 5 |
| 7 | 3 | 1 | 4 |
| 8 | 3 | 2 | 5 |
| 9 | 3 | 3 | 5 |
id_cat 2 and 3 should have the same "row" and "col_md" values as in id_cat=1.
I've tried with this post first answer like this:
UPDATE `myTable` AS t1 JOIN `myTable` AS t2 ON t2.id_cat=1
SET t1.row = t2.row, t1.col_md = t2.col_md
WHERE t1.id_cat = 2 or t1.id_cat=3;
but that results on all "row" column values equal to 1.
What I'm doing wrong and what's the way to do this right?
EDIT:
The tables above are just examples to make this ask easier to understand, but the real table is bigger (4k rows) and:
"row" column with id_cat=1 can have any number and not a sequence as in the example.
"col_md" columns can have any number too.
That's why the update must set a copy of the id_cat=1 "row" and "col_md" values in the id_cat!=1 "row" and "col_md" values.
If this can't be done with just MySQL, a php script will be nice too.

Answer 1

In the example query you gave, you are updating t1.row with t2.row. As you are joining on the id_cat, this will result in multiple rows selected to update a single row, so the outcome just takes the first row.
What you actually want, is to make the 1-to-1 relation in the update, so what needs to be changed in your query is to add the row matching in the join and remove the assignment in the SET, like this:
UPDATE `myTable` AS t1 JOIN `myTable` AS t2 ON t2.id_cat=1 AND t1.row = t2.row
SET t1.col_md = t2.col_md
WHERE t1.id_cat = 2 or t1.id_cat=3;
Which then gives the output of:
MariaDB [testart]> select * from myTable;
+------+--------+------+--------+
| id | id_cat | row | col_md |
+------+--------+------+--------+
| 1 | 1 | 1 | 4 |
| 2 | 1 | 2 | 5 |
| 3 | 1 | 3 | 5 |
| 4 | 2 | 1 | 4 |
| 5 | 2 | 2 | 5 |
| 6 | 2 | 3 | 5 |
| 7 | 3 | 1 | 4 |
| 8 | 3 | 1 | 4 |
| 9 | 3 | 2 | 5 |
+------+--------+------+--------+
9 rows in set (0.00 sec)

Answer 2

Currently able to achieve the SQL query for your desired result.
SELECT t2.id_cat, t1.row, t1.col_md
FROM (SELECT row, col_md from mytable WHERE id_cat=1) as t1 , mytable as t2
GROUP BY t2.id_cat, t1.row, t1.col_md
The above will return the following..
I suggest to use INSERT statement along with the above query to put the record into a new table and drop the old one.
Cheers!
EDITED...
Instead of Updating table, alternate approach could be to Insert the required record into a new table.
This can be achieved with following four steps
Create a tmp table with same fileds (id Auto_Increment, id_cat, row, col_md)
Insert to tmp table with this statement...
INSERT INTO tmp(id_cat, row, col_md)
SELECT t2.id_cat, t1.row, t1.col_md
FROM (SELECT row, col_md from mytable WHERE id_cat=1) as t1 , mytable as t2
GROUP BY t2.id_cat, t1.row, t1.col_md
Remove/Rename 'myTable'.
Rename 'tmp' table to 'myTable'.
Hope this will serve the purpose...
Cheers!

Answer 3

it's not enough to tell which group you want the data from, you need to match id to id.
in your case t2.id 4 and 7 to t1.id 1, t2.id 5 and 8 to t1.id 2, and t2.id 6 and 9 to t1.id 3.
SELECT #d := COUNT(*) FROM myTable WHERE id_cat = 1;
UPDATE `myTable` AS t1
JOIN `myTable` AS t2 ON t2.id_cat=1 AND
t2.id = IFNULL(NULLIF(t1.id MOD #d, 0), #d)
SET t1.row = t2.row, t1.col_md = t2.col_md
WHERE t1.id_cat = 2 or t1.id_cat=3;
#d holds the number of lines where id_cat = 1
we divide t1.id by #d and match the remainder (MOD) to t2.id.
when t1.id is multiple of #d the remainder is 0 and we have to match it to #d
so we make 0 into NULL and NULL into #d

Answer 4

In my understanding, the difficult part about this question is to relate each record to update (ie each record with id_cat IN (2, 3)) to the relevant original record (record with id_cat = 1).
Based on your sample data, I understand that you expect series of records for each id_cat (I can see three groups of three records, sorted by increasing id), so I would assume that you want to relate each record to the original that has the same sequence in the group of record where id_cat = 1.
Assuming MySQL 8.0, a typical approach to assign a number to a record within a group is ROW_NUMBER(). Consider this simple query:
SELECT
t.*,
ROW_NUMBER() OVER(PARTITION BY id_cat ORDER BY id) rn
FROM t
Yields:
| id | id_cat | rw | col_md | rn |
| --- | ------ | --- | ------ | --- |
| 1 | 1 | 1 | 4 | 1 |
| 2 | 1 | 2 | 5 | 2 |
| 3 | 1 | 3 | 5 | 3 |
| 4 | 2 | 1 | 3 | 1 |
| 5 | 2 | 2 | 4 | 2 |
| 6 | 2 | 2 | 4 | 3 |
| 7 | 3 | 1 | 12 | 1 |
| 8 | 3 | 1 | 12 | 2 |
| 9 | 3 | 2 | 3 | 3 |
Now with this set-up in mind, we can turn this query to a Common Table Expression (available also starting MySQL 8.0), and JOIN it as need with the original table to do the UPDATE:
WITH cte AS (
SELECT
t.*,
ROW_NUMBER() OVER(PARTITION BY id_cat ORDER BY id) rn
FROM t
)
UPDATE t t0
INNER JOIN cte t1 ON t1.id = t0.id
INNER JOIN cte t2 ON t2.id_cat = 1 AND t2.rn = t1.rn
SET t0.rw = t2.rw, t0.col_md = t2.col_md
WHERE t0.id_cat IN (2, 3)
Details:
t0 is the original table, where records having id_cat IN (2, 3) need to be updated
t1 is the corresponding record in the CTE (to which a row number was assigned)
t2 is the record in the CTE that has id_cat = 1 and the same row number as the record being updated
Demo on DB Fiddle:
| id | id_cat | rw | col_md |
| --- | ------ | --- | ------ |
| 1 | 1 | 1 | 4 |
| 2 | 1 | 2 | 5 |
| 3 | 1 | 3 | 5 |
| 4 | 2 | 1 | 4 |
| 5 | 2 | 2 | 5 |
| 6 | 2 | 3 | 5 |
| 7 | 3 | 1 | 4 |
| 8 | 3 | 2 | 5 |
| 9 | 3 | 3 | 5 |