Selecting multiple columns from multiple rows in one column (subquery?)

I'm working with MySQL 5.1.68.

I have a situation where I'd like to select rows from another table into a column.

I'd like to output this:

id # name # citizen_name (multiple citizen names)

1 # The Hague # Barack Obama, Marc Zuckerberg, George Bush

I know how to do this with multiple queries, but I'd like to do this with just one query. I've been looking at group_concat and concat_ws, but unfortunatly this hasn't brough me the result I was looking for. I've also tried to put the results from a SELECT sub query into a variable and CONCAT those results, but that didin't work either.

My SQL fiddle to show the schema and some test data: http://sqlfiddle.com/#!2/c7c7e/1.

What can I do?

You need to use GROUP_CONCAT to aggregate the citizen names first. Then use CONCAT

SELECT CONCAT(id,' # ',name,' # ',citizens) listing FROM
(
    SELECT A.id,A.name,GROUP_CONCAT(B.name,' ',B.surname) citizens
    FROM city A INNER JOIN citizen B
    ON A.id = B.city_id GROUP BY A.id,A.name
) AA;

Here is the SQL Fiddle to prove it : http://sqlfiddle.com/#!2/9f4b3/1

If the names become too long due to truncation, you will have to extend GROUP_CONCAT's maximum length. To set it to 16K, run the following:

SET group_concat_max_len = 1024 * 16;
SET GLOBAL group_concat_max_len = 1024 * 16;

First line is for the session, the second is for all new DB Connections.

StackExchange Database Administrators Q#52441, answer score: 2