Concatenate several fields into one with SQL - sql

Question

I have three tables tag, page, pagetag
With the data below
page
ID NAME
1 page 1
2 page 2
3 page 3
4 page 4
tag
ID NAME
1 tag 1
2 tag 2
3 tag 3
4 tag 4
pagetag
ID PAGEID TAGID
1 2 1
2 2 3
3 3 4
4 1 1
5 1 2
6 1 3
I would like to get a string containing the correspondent tag names for each page with SQL in a single query. This is my desired output.
ID NAME TAGS
1 page 1 tag 1, tag 2, tag 3
2 page 2 tag 1, tag 3
3 page 3 tag 4
4 page 4
Is this possible with SQL?
I am using MySQL. Nonetheless, I would like a database vendor independent solution if possible.

Answer 1

Sergio del Amo:
However, I am not getting the pages without tags. I guess i need to write my query with left outer joins.
SELECT pagetag.id, page.name, group_concat(tag.name)
FROM
(
page LEFT JOIN pagetag ON page.id = pagetag.pageid
)
LEFT JOIN tag ON pagetag.tagid = tag.id
GROUP BY page.id;
Not a very pretty query, but should give you what you want - pagetag.id and group_concat(tag.name) will be null for page 4 in the example you've posted above, but the page shall appear in the results.

Answer 2

Yep, you can do it across the 3 something like the below:
SELECT page_tag.id, page.name, group_concat(tags.name)
FROM tag, page, page_tag
WHERE page_tag.page_id = page.page_id AND page_tag.tag_id = tag.id;
Has not been tested, and could be probably be written a tad more efficiently, but should get you started!
Also, MySQL is assumed, so may not play so nice with MSSQL! And MySQL isn't wild about hyphens in field names, so changed to underscores in the above examples.

Answer 3

As far as I'm aware SQL92 doesn't define how string concatenation should be done. This means that most engines have their own method.
If you want a database independent method, you'll have to do it outside of the database.
(untested in all but Oracle)
Oracle
SELECT field1 | ', ' | field2
FROM table;
MS SQL
SELECT field1 + ', ' + field2
FROM table;
MySQL
SELECT concat(field1,', ',field2)
FROM table;
PostgeSQL
SELECT field1 || ', ' || field2
FROM table;

Answer 4

I got a solution playing with joins. The query is:
SELECT
page.id AS id,
page.name AS name,
tagstable.tags AS tags
FROM page
LEFT OUTER JOIN
(
SELECT pagetag.pageid, GROUP_CONCAT(distinct tag.name) AS tags
FROM tag INNER JOIN pagetag ON tagid = tag.id
GROUP BY pagetag.pageid
)
AS tagstable ON tagstable.pageid = page.id
GROUP BY page.id
And this will be the output:
id name tags
---------------------------
1 page 1 tag2,tag3,tag1
2 page 2 tag1,tag3
3 page 3 tag4
4 page 4 NULL
Is it possible to boost the query speed writing it another way?

Answer 5

pagetag.id and group_concat(tag.name) will be null for page 4 in the example you've posted above, but the page shall appear in the results.
You can use the COALESCE function to remove the Nulls if you need to:
select COALESCE(pagetag.id, '') AS id ...
It will return the first non-null value from it's list of parameters.

Answer 6

I think you may need to use multiple updates.
Something like (not tested):
select ID as 'PageId', Name as 'PageName', null as 'Tags'
into #temp
from [PageTable]
declare #lastOp int
set #lastOp = 1
while #lastOp > 0
begin
update p
set p.tags = isnull(tags + ', ', '' ) + t.[Tagid]
from #temp p
inner join [TagTable] t
on p.[PageId] = t.[PageId]
where p.tags not like '%' + t.[Tagid] + '%'
set #lastOp == ##rowcount
end
select * from #temp
Ugly though.
That example's T-SQL, but I think MySql has equivalents to everything used.