use MySQL to update fields with group - mysql

Question

id item_id item_order
--- ---------- -----------
1 1 0
2 1 0
3 1 0
4 2 0
5 2 0
6 2 0
7 3 0
8 3 0
9 3 0
10 3 0
How to use MySQL statement to update the order fields to:
id item_id item_order
--- ---------- -----------
1 1 1
2 1 2
3 1 3
4 2 1
5 2 2
6 2 3
7 3 1
8 3 2
9 3 3
10 3 4

Answer 1

Subquery can provide an elegance solution. Assuming your table is called test:
UPDATE test a SET item_order = (
SELECT count(*) FROM ( SELECT id, item_id FROM test ) b
WHERE a.item_id = b.item_id AND b.id <= a.id
)
How it works? For each row it would query the number of rows with same item_id and lower or equal id. This may be not as scalable as other solutions, I haven't tested, but for smaller tables this is easier to understand and update.
Notice that b is another subquery. This would force MySQL to create an internal temp table so that it won't complain we are reading from the table we are updating.

Answer 2

do this
create table tmp
(
id int(10) unsigned,
`order` int(10) unsigned auto_increment,
... // any other columns
primary key (id, `order`)
);
insert into tmp select * from old_table; // you can specify order if you want to
after getting the order correctly,
you can then remove the primary key on two columns
NOTE -- avoid reserved words as column name at all cost
IT WORKS
mysql> select * from test;
+------+--------+
| id | orders |
+------+--------+
| 1 | 0 |
| 1 | 0 |
| 1 | 0 |
| 2 | 0 |
| 2 | 0 |
| 2 | 0 |
| 2 | 0 |
+------+--------+
7 rows in set (0.00 sec)
mysql> insert into tmp select * from test;
Query OK, 7 rows affected (0.00 sec)
Records: 7 Duplicates: 0 Warnings: 0
mysql> select * from tmp;
+----+--------+
| id | orders |
+----+--------+
| 1 | 1 |
| 1 | 2 |
| 1 | 3 |
| 2 | 1 |
| 2 | 2 |
| 2 | 3 |
| 2 | 4 |
+----+--------+
7 rows in set (0.00 sec)
mysql> desc tmp;
+--------+---------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------+---------+------+-----+---------+----------------+
| id | int(10) | NO | PRI | 0 | |
| orders | int(10) | NO | PRI | NULL | auto_incrent |
+--------+---------+------+-----+---------+----------------+
2 rows in set (0.00 sec)

Answer 3

Select version:
select id,item_id,
#num:= if(#item_id=item_id,#num+1,1) as posit,
#item_id:=item_id as itid
from tab,(select #num:=0,#item_id:=0) as t order by item_id,id
+----+---------+-------+------+
| id | item_id | posit | itid |
+----+---------+-------+------+
| 1 | 1 | 1 | 1 |
| 2 | 1 | 2 | 1 |
| 3 | 1 | 3 | 1 |
| 4 | 2 | 1 | 2 |
| 5 | 2 | 2 | 2 |
| 6 | 2 | 3 | 2 |
| 7 | 3 | 1 | 3 |
| 8 | 3 | 2 | 3 |
| 9 | 3 | 3 | 3 |
| 10 | 3 | 4 | 3 |
+----+---------+-------+------+
10 rows in set (0.33 sec)
Update version:
update tab as t1
inner join (
select id,item_id,
#num:= if(#item_id=item_id,#num+1,1) as posit,
#item_id:=item_id as itid
from tab,(select #num:=0,#item_id:=0) as t order by item_id,id )
as t2
set t1.item_order = t2.posit
where t1.id = t2.id
+----+---------+------------+
| id | item_id | item_order |
+----+---------+------------+
| 1 | 1 | 1 |
| 2 | 1 | 2 |
| 3 | 1 | 3 |
| 4 | 2 | 1 |
| 5 | 2 | 2 |
| 6 | 2 | 3 |
| 7 | 3 | 1 |
| 8 | 3 | 2 |
| 9 | 3 | 3 |
| 10 | 3 | 4 |
+----+---------+------------+
10 rows in set (0.00 sec)