how to create a counting column for a particular item - mysql

Question

I have the following schema (mysql)
create table test(
userid int(11) not null,
item varchar(15),
bookid int(11));
insert into test values ('1','journal',NULL);
insert into test values ('1','journal',NULL);
insert into test values ('1','book',NULL);
insert into test values ('2','book',NULL);
insert into test values ('2','journal',NULL);
insert into test values ('1','book',NULL);
insert into test values ('2','journal',NULL);
insert into test values ('3','book',NULL);
insert into test values ('1','book',NULL);
insert into test values ('1','journal',NULL);
insert into test values ('3','journal',NULL);
insert into test values ('1','journal',NULL);
insert into test values ('2','journal',NULL);
insert into test values ('2','book',NULL);
insert into test values ('2','journal',NULL);
insert into test values ('1','journal',NULL);
insert into test values ('3','book',NULL);
insert into test values ('3','book',NULL);
insert into test values ('3','book',NULL);
insert into test values ('3','book',NULL);
whenever there is a book, I'm trying assign an auto increment beginning with 1 in the bookid column. For each user, the numbering begins again from 1. I know a way this can be done by creating a separate table. Is there a way I can avoid that and accomplish that using some sort of update query in this very table and update the column bookid? I am trying to get output similar to the following:
userid,item,bookid
'1','journal',NULL
'1','journal',NULL
'1','book',1
'2','book',1
'2','journal',NULL
'1','book',2
'2','journal',NULL
'3','book',1
'1','book',3
'1','journal',NULL
'3','journal',NULL
'1','journal',NULL
'2','journal',NULL
'2','book',2
'2','journal',NULL
'1','journal',NULL
'3','book',2
'3','book',3
'3','book',4
'3','book',5
I appreciate if someone could guide me on how to accomplish this?

Answer 1

Here's one idea...
drop table if exists test;
create table test
(id INT NOT NULL AUTO_INCREMENT PRIMARY KEY
,userid int not null
,item varchar(15) NOT NULL
);
insert into test (userid,item) values
(1,'journal')
,(1,'journal')
,(1,'book')
,(2,'book')
,(2,'journal')
,(1,'book')
,(2,'journal')
,(3,'book')
,(1,'book')
,(1,'journal')
,(3,'journal')
,(1,'journal')
,(2,'journal')
,(2,'book')
,(2,'journal')
,(1,'journal')
,(3,'book')
,(3,'book')
,(3,'book')
,(3,'book');
SELECT x.*
, COUNT(*) rank
FROM test x
JOIN test y
ON y.userid = x.userid
AND y.item = x.item
AND y.id <= x.id
GROUP
BY id
ORDER
BY userid
, item
, rank;
+----+--------+---------+------+
| id | userid | item | rank |
+----+--------+---------+------+
| 3 | 1 | book | 1 |
| 6 | 1 | book | 2 |
| 9 | 1 | book | 3 |
| 1 | 1 | journal | 1 |
| 2 | 1 | journal | 2 |
| 10 | 1 | journal | 3 |
| 12 | 1 | journal | 4 |
| 16 | 1 | journal | 5 |
| 4 | 2 | book | 1 |
| 14 | 2 | book | 2 |
| 5 | 2 | journal | 1 |
| 7 | 2 | journal | 2 |
| 13 | 2 | journal | 3 |
| 15 | 2 | journal | 4 |
| 8 | 3 | book | 1 |
| 17 | 3 | book | 2 |
| 18 | 3 | book | 3 |
| 19 | 3 | book | 4 |
| 20 | 3 | book | 5 |
| 11 | 3 | journal | 1 |
+----+--------+---------+------+
Note that MyISAM actually lets you use a composite PK in which part of that composite is an auto-incrementing id, but InnoDB prohinits this.
On larger datasets a query along these lines will likely be far more efficient...
SELECT id
, userid
, item
, IF(#userid=userid,IF(#item=item,#i:=#i+1,#i:=1),#i:=1) rank
, #userid := userid
, #item := item
FROM test
, (SELECT #userid = NULL,#item:='',#i:=1) vars
ORDER
BY userid,item,id;