Make new column which is incremented by it's order - mysql

Question

I need to make new column for my table Products -> called Order (new column). And using rails migration I need to add new column and instantly set it's order number, but it's need to be done by product_id.
What I mean I need something like:
product_id | order
1 ------------> 1
1 ------------> 2
1 ------------> 3
2 ------------> 1
2 ------------> 2
Is there a way of doing it?
EDIT :
You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ''order' = t1.'order'' at line 15:
update product_submissions t
join (
select
id,
product_id,
'order' from (
select id,
product_id,
#rn:= if(#prev = product_id,#rn:=#rn+1,1) as 'order',
#prev:=product_id
from product_submissions,
(select #rn:=0,#prev:=0)r
order by product_id,id
)x
)t1 on t1.id=t.id set t.'order' = t1.'order'

Answer 1

Consider the following
mysql> create table test (id int ,product_id int);
Query OK, 0 rows affected (0.14 sec)
mysql> insert into test values (1,1),(2,1),(3,1),(4,2),(5,2);
Now lets create the order
select
product_id,
`order` from (
select
product_id,
#rn:= if(#prev = product_id,#rn:=#rn+1,1) as `order`,
#prev:=product_id from test,(select #rn:=0,#prev:=0)r
order by product_id,id
)x ;
This will give you something as
+------------+-------+
| product_id | order |
+------------+-------+
| 1 | 1 |
| 1 | 2 |
| 1 | 3 |
| 2 | 1 |
| 2 | 2 |
+------------+-------+
Now lets use in update command, but before that lets add the column (in your case its already there)
mysql> alter table test add column `order` int ;
Query OK, 5 rows affected (0.29 sec)
mysql> select * from test ;
+------+------------+-------+
| id | product_id | order |
+------+------------+-------+
| 1 | 1 | NULL |
| 2 | 1 | NULL |
| 3 | 1 | NULL |
| 4 | 2 | NULL |
| 5 | 2 | NULL |
+------+------------+-------+
5 rows in set (0.00 sec)
Finally the update command
update test t
join (
select
id,
product_id,
`order` from (
select id,
product_id,
#rn:= if(#prev = product_id,#rn:=#rn+1,1) as `order`,
#prev:=product_id
from test,(select #rn:=0,#prev:=0)r
order by product_id,id
)x
)t1 on t1.id=t.id set t.`order` = t1.`order`
mysql> select * from test ;
+------+------------+-------+
| id | product_id | order |
+------+------------+-------+
| 1 | 1 | 1 |
| 2 | 1 | 2 |
| 3 | 1 | 3 |
| 4 | 2 | 1 |
| 5 | 2 | 2 |
+------+------------+-------+
5 rows in set (0.00 sec)