Re: Need a SQL to create sets of hobbies - Mailing list pgsql-sql
| From | Aaron Bono |
|---|---|
| Subject | Re: Need a SQL to create sets of hobbies |
| Date | |
| Msg-id | bf05e51c0609201138t6e453c5pf85863a3a422cf08@mail.gmail.com Whole thread Raw |
| In response to | Need a SQL to create sets of hobbies ("CN" <cnliou9@fastmail.fm>) |
| List | pgsql-sql |
On 9/20/06, CN <cnliou9@fastmail.fm> wrote:
Your best design is to break this into 3 tables:
person (
person_id,
person_name
)
hobby (
hobby_id,
hobby_name
)
person_hobby (
person_id,
hobby_id
)
Then you can get the list of hobbies for each person like this:
SELECT
person.person_id,
array(
SELECT hobby_name
FROM person_hobby
INNER JOIN hobby ON (hobby.hobby_id = person_hobby.hobby_id)
WHERE person_hobby.person_id = person.person_id
ORDER BY hobby_name
) AS hobby_list
FROM person
Then do this to flatten it back out:
SELECT
-- Gives you a unique id though using a SERIAL on a table would be better
min(mysub.person_id),
hobby.hobby_name
FROM (
SELECT
person.person_id,
array(
SELECT hobby_name
FROM person_hobby
INNER JOIN hobby ON (hobby.hobby_id = person_hobby.hobby_id)
WHERE person_hobby.person_id = person.person_id
ORDER BY hobby_name
) AS hobby_list
FROM person
) mysub
INNER JOIN person_hobby ON (person_hobby.person_id = mysub.person_id)
INNER JOIN hobby ON (hobby.hobby_id = person_hobby.hobby_id)
GROUP BY
mysub.hobby_list,
hobby.hobby_name
I did not try it so it may require a little tweaking to work. Also, I don't know what the performance would be like.
==================================================================
Aaron Bono
Aranya Software Technologies, Inc.
http://www.aranya.com
http://codeelixir.com
==================================================================
Hi!
CREATE TABLE x1 (name TEXT, hobby TEXT, PRIMARY KEY(name,hobby));
INSERT INTO x1 VALUES('John','music');
INSERT INTO x1 VALUES('John','arts');
INSERT INTO x1 VALUES('Bob','arts');
INSERT INTO x1 VALUES('Bob','music');
INSERT INTO x1 VALUES('Rocky','copmputer');
INSERT INTO x1 VALUES('Steve','arts');
INSERT INTO x1 VALUES('Steve','football');
INSERT INTO x1 VALUES('Tom','computer');
INSERT INTO x1 VALUES('Tom','music');
select * from x1;
name | hobby
-------+----------
John | music
John | arts
Bob | arts
Bob | music
Rocky | computer
Steve | arts
Steve | football
Tom | computer
Tom | music
(9 rows)
John and Bob have the same hobbies - music and arts. So music and arts
are treated as one set of hobbies.
Rocky has an unique set of interest - computer.
Steve also likes arts just as John and Bob do, but he also has an
exclusive interest - football. Thus, his set of hobbies is unique -
arts, football.
One of Tom's hobbies, music, overlaps those of John, Bob, and Rocky; but
computer does not. Hence his hobbies, computer and music, forms a new
set of hobbies.
Now we have 4 sets of hobbies:
set 1: music, arts
set 2: computer
set 3: arts, football
set 4: computer, music
I am looking for an SQL that creates sets of hobbies in table x2 by
selecting from table x1:
CREATE TABLE x2 (sid INTEGER, hobby TEXT, PRIMARY KEY(sid,hobby));
and makes x2 contain rows:
sid | hobby
-------+----------
1 | music
1 | arts
2 | computer
3 | arts
3 | football
4 | computer
4 | music
where gid starts from 1.
person (
person_id,
person_name
)
hobby (
hobby_id,
hobby_name
person_hobby (
person_id,
hobby_id
)
Then you can get the list of hobbies for each person like this:
SELECT
person.person_id,
array(
SELECT hobby_name
FROM person_hobby
INNER JOIN hobby ON (hobby.hobby_id = person_hobby.hobby_id)
WHERE person_hobby.person_id = person.person_id
ORDER BY hobby_name
) AS hobby_list
FROM person
Then do this to flatten it back out:
SELECT
-- Gives you a unique id though using a SERIAL on a table would be better
min(mysub.person_id),
hobby.hobby_name
FROM (
SELECT
person.person_id,
array(
SELECT hobby_name
FROM person_hobby
INNER JOIN hobby ON (hobby.hobby_id = person_hobby.hobby_id)
WHERE person_hobby.person_id = person.person_id
ORDER BY hobby_name
) AS hobby_list
FROM person
) mysub
INNER JOIN person_hobby ON (person_hobby.person_id = mysub.person_id)
INNER JOIN hobby ON (hobby.hobby_id = person_hobby.hobby_id)
GROUP BY
mysub.hobby_list,
hobby.hobby_name
I did not try it so it may require a little tweaking to work. Also, I don't know what the performance would be like.
==================================================================
Aaron Bono
Aranya Software Technologies, Inc.
http://www.aranya.com
http://codeelixir.com
==================================================================