I am trying to write a MySQL query that retrieves one record from table "projects" that has a one-to-many relationship with table "tags". My application uses 4 tables to do this:
Projects - the projects table
Entities - entity table; references several application resources
Tags - tags table
Tag_entity - links tags to entities
Is it possible to write the query in such a way that multiple values from table "Tags" are concatenated into one result column? I'd prefer doing this without using subqueries.
Table clarification:
-------------
| Tag_Entity |
------------- ---------- | ----------- | -------
| Projects | | Entities | | - id | | Tags |
| ----------- | | -------- | | - tag_id | | ----- |
| - id | --> | - id | --> | - entity_id | --> | id |
| - entity_id | ---------- ------------- | name |
------------- -------
Desired result:
Projects.id Entities.id Tags.name (concatenated)
1 5 'foo','bar','etc'
see GROUP_CONCAT
example:
mysql> SELECT * FROM blah;
+----+-----+-----------+
| K | grp | name |
+----+-----+-----------+
| 1 | 1 | foo |
| 2 | 1 | bar |
| 3 | 2 | hydrogen |
| 4 | 4 | dasher |
| 5 | 2 | helium |
| 6 | 2 | lithium |
| 7 | 4 | dancer |
| 8 | 3 | winken |
| 9 | 4 | prancer |
| 10 | 2 | beryllium |
| 11 | 1 | baz |
| 12 | 3 | blinken |
| 13 | 4 | vixen |
| 14 | 1 | quux |
| 15 | 4 | comet |
| 16 | 2 | boron |
| 17 | 4 | cupid |
| 18 | 4 | donner |
| 19 | 4 | blitzen |
| 20 | 3 | nod |
| 21 | 4 | rudolph |
+----+-----+-----------+
21 rows in set (0.00 sec)
mysql> SELECT grp, GROUP_CONCAT(name ORDER BY K) FROM blah GROUP BY grp;
+-----+----------------------------------------------------------------+
| grp | GROUP_CONCAT(name ORDER BY K) |
+-----+----------------------------------------------------------------+
| 1 | foo,bar,baz,quux |
| 2 | hydrogen,helium,lithium,beryllium,boron |
| 3 | winken,blinken,nod |
| 4 | dasher,dancer,prancer,vixen,comet,cupid,donner,blitzen,rudolph |
+-----+----------------------------------------------------------------+
4 rows in set (0.00 sec)
I don't know if it works in MySQL, but in SQL Server you can use a trick for this:
DECLARE @csv varchar(max)
SET @csv = ''
SELECT @csv = @csv + ',' + foo.SomeColumn
FROM [FOO] foo
WHERE foo.SomeId = @SomeId
Then in the main select
SELECT ..., @csv AS [Tags]
FROM ...
The result of SELECT @csv = @csv + ',' + foo.SomeColumn line is that @csv becomes the comma-separated list of all matching records from the source table (after predicate).
Worth trying in MySQL?
