Need help with a complex SQL query - I think I need a two-stage inner join or something like that? - sql

Question

Okay, here's what I'm trying to do. I have a drupal table (term_data) in mysql that lists tags and their ID numbers and a second table, (term_node) that shows the relationship between tags and the data the tags refer to. For example, if node 1 had 3 tags, "A", "B" and "C". term_data might look like this:
name tid
A 1
B 2
C 3
and term_node might look like this:
nid tid
1 1
1 2
2 2
3 3
3 2
In this example, node 1 has been tagged with "A" and "B", node 2 has been tagged with "A" and node 3 has been tagged with "B", and "C".
I need to write a query that, given a tag name, list for me all the OTHER tags that are ever used with that tag. In the above example, searching on "A" should return "A" and "B" because node 1 uses both, searching on "C" should return "B" and "C", and searching on "B" should return "A", "B" and "C".
Any ideas? I got this far:
select distinct n.nid from term_node n INNER join term_data t where n.tid = t.tid and t.name='A';
Which gives me a list of every node that has been tagged with "A" - but I can't figure out the next step.
Can anyone help me out?

Answer 1

Try:
select distinct d2.name
from term_data d1
join term_node n1 on d1.tid = n1.tid
join term_node n2 on n1.nid = n2.nid
join term_data d2 on n2.tid = d2.tid
where d1.name = 'A'

Answer 2

Updated: Mark pointed out that the query wasn't correct.
SELECT DISTINCT t.name, t2.name Other
FROM
term_data t
INNER JOIN term_node n ON t.tid = n.tid
INNER JOIN term_node n2 ON n2.nid = n.nid
INNER JOIN term_data t2 ON n2.tid = t2.tid
WHERE
t.name = 'A'
Marks answer should be accepted since he got it right first. Here is a demonstration of a similar query
https://data.stackexchange.com/stackoverflow/query/13283/demo-for-need-help-with-a-complex-sql-query

Answer 3

Your description of term_node data and the example do not seem to match but using the example data provided I believe the following query will do what you need.
select distinct td.name, td2.name as tagged_name
from term_data td
inner join term_node tn
on tn.tid = td.tid
inner join term_node tn2
on tn2.nid = tn.nid
inner join term_data td2
on td2.tid = tn2.tid
The first join looks up the term_node records that match the name, term_node is then joined to itself to find all other tid's for that node, finally the second term_node is joined to term_data to retrieve the names of the tag.
You need to tack on the appropriate where clause to select just the tag you want.
Result set follows for above:-
name tagged_name
A A
A B
B A
B B
B C
C B
C C
Hope this helps
Ray

Answer 4

I created the schema in my workbench, and here's the query I came up with:
SELECT * FROM `term_data` WHERE `term_data`.`tid` IN (
SELECT `term_node`.`tid` from `term_node` WHERE `nid` IN (
SELECT `nid` FROM `term_node` JOIN `term_data` ON `term_data`.`tid` = `term_node`.`tid` WHERE `term_data`.`name` = 'A'
)
);
Sorry for the structure ;) Here's SHOW CREATE TABLE for both tables:
CREATE TABLE `term_data` (
`tid` int(11) NOT NULL,
`name` varchar(45) DEFAULT NULL,
PRIMARY KEY (`tid`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1
CREATE TABLE `term_node` (
`term_node_id` int(11) NOT NULL,
`nid` int(11) NOT NULL,
`tid` varchar(45) DEFAULT NULL,
PRIMARY KEY (`term_node_id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1
This seemed to work as expected, if I understood your question correctly. So one more time, we have some nodes which are tagged. We'd like to select a tag (A), and then select other tags that were used to tag same nodes as tag A.
Cheers.
P.S. Output is the following:
tid name
/* For tag A */
1 A
2 B
/* For tag B */
1 A
2 B
3 C
/* For tag C */
2 B
3 C