Selecting rows with reference id's to same table from mysql - mysql

Question

I have a table such as:
id name ref_id order data_obj
-- ---- ------ ----- --------
1 Sam 0 15 [binary data]
2 Jack 0 20 [binary data]
3 Sue 0 25 [binary data]
4 Sam2 1 - [no data]
5 Sue2 3 - [no data]
6 Sam3 1 - [no data]
The idea is that I have more columns other than data_obj which can be common, so I don't want to insert them again, just want to insert a reference id to the same data.
Is it possible to write a query and select this:
1 - Sam - binary data from id 1
4 - Sam2 - binary data from id 1
6 - Sam3 - binary data from id 1
2 - Jack - binary data from id 2
3 - Sue - binary data from id 3
5 - Sue2 - binary data from id 3
Please note that I'm ordering according to column named order and there's no actual data for this column for referenced rows.

Answer 1

SELECT t1.id, t1.name, t2.data_obj
FROM your_table t1
LEFT JOIN your_table t2 ON t1.ref_id = t2.id
ORDER BY t1.order
Other version, which doesn't return rows without ref
SELECT t1.id, t1.name, t2.data_obj
FROM your_table t1, your_table t2
WHERE t1.ref_id = t2.id
ORDER BY t1.order

Answer 2

Here's a modification of #vartec's answer. This modification uses COALESCE() to combine the data_obj from either the primary row or the referenced row.
SELECT t1.id, t1.name, COALESCE(t1.data_obj, t2.data_obj)
FROM your_table t1
LEFT JOIN your_table t2 ON t1.ref_id = t2.id
ORDER BY COALESCE(t1.order, t2.order), ref_id;
COALESCE() is a standard SQL function that returns its first non-NULL argument.

Answer 3

Why aren't you using more than one table?
CREATE TABLE user (
user_id number not null (some form of auto increment or sequence),
name varchar(50) not null,
otherdata type,
primary key (id));
CREATE TABLE common (
common_id number not null (autoinc),
user_id number not null,
commondata type,
primary key (common_id),
unique index (user_id, common_id));
SELECT u.name, u.otherdata, c.commondata
FROM user u, common c
WHERE u.user_id = c.user_id
TABLE user
user_id name otherdata
1 Sam abc
2 Jack def
3 Sue ghi
Table common
common_id user_id commondata
1 1 AAA
2 1 BBB
3 1 CCC
4 2 DDD
5 3 EEE
6 3 FFF
Output
name otherdata commondata
Sam abc AAA
Sam abc BBB
Sam abc CCC
Jack def DDD
Sue ghi EEE
Sue ghi FFF