How to prevent GROUP_CONCAT from creating a result when no input data is present?

Question

Given the following MySQL query:

SELECT
  `show`.`id`
  , GROUP_CONCAT( `showClips`.`clipId` ORDER BY `position` ASC ) AS 'playlist'
FROM
  `show`
  INNER JOIN
    `showClips`
      ON
        ( `show`.`id` = `showClips`.`showId` )
;

I want to retrieve a list of all "shows" from the database, including the ids of contained "clips".

This works fine, as long as there are entries in the show table. For this problem, let's assume all tables are completely empty.

GROUP_CONCAT will return NULL and thus forcing a row into the result (which contains only NULL values).

My application will then think that one show/result exists. But that result will be invalid. This can of course be checked, but I feel like this could (and should) be prevented in the query already.

Answer 1

You should simply add a GROUP BY at the end.

Test case:

CREATE TABLE `show` (id int);
CREATE TABLE `showClips` (clipId int, showId int, position int);

SELECT 
   `show`.`id`,
   GROUP_CONCAT( `showClips`.`clipId` ORDER BY `position` ASC ) AS 'playlist'
FROM  `show`
INNER JOIN `showClips` ON ( `show`.`id` = `showClips`.`showId` )
GROUP BY `show`.`id`;

Empty set (0.00 sec)

Answer 2

Add group by show.id, then result will be correct for empty tables:

create table emptyt(id int, name varchar(20));

   select id, group_concat(name) from emptyt

result:

 NULL, NULL

query with group by

 select id, group_concat(name) from emptyt
 group by Id

result:

empty dataset