当没有输入数据时,如何防止组CONCAT创建结果?

GROUP BY

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)

添加分组依据 show . id

创建表emptyt(id int,name varchar(20));

   select id, group_concat(name) from emptyt

 NULL, NULL

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

结果:

空数据集