How-To: Aggregate data from multiple rows into a delimited list.

This is not a question, but a solution. I just wanted to share this
with others on the list in case it saves you a few hours of searching...

I wanted to select several rows of data and have them returned in a
single record with the rows joined by a delimiter. Turns out this is
very easy to do in PostgreSQL:

SELECT a.id, a.name,
ARRAY_TO_STRING(ARRAY(
SELECT b.name
FROM b
WHERE b.id = a.id
ORDER BY b.name ASC
), ',') AS b_names
FROM a
ORDER BY a.id ASC;

Sample data would look like this:

[table a]
id | name
----+------
1 | one
2 | two
3 | three
4 | four

[table b]
id | name
----+------
1 | pizza
1 | hot dog
2 | gorilla
2 | monkey
3 | apple
4 | cheese
4 | milk
4 | eggs

And the result would look like this:

id | name | b_names
----+-------+---------
1 | one | pizza,hot dog
2 | two | gorilla,monkey
3 | three | apple
4 | four | cheese,milk,eggs

This is an easy way to return attributes of a record from another table
without having to issue multiple queries or deal with multiple result
records.

Enjoy!

-- Dante