SELECT
    `devices`.`id`, `projects`.`data`
FROM  
    `devices`
    LEFT JOIN `projects` ON FIND_IN_SET(`devices`.`idshelf`, `projects`.`idshelf`) AND `projects`.`finished` = 0

MariaDB [(none)]> SELECT 2 IN(1,2,3);
+-------------+
| 2 IN(1,2,3) |
+-------------+
|           1 |
+-------------+
1 row in set (0.00 sec)

MariaDB [(none)]> SELECT 2 IN('1,2,3');
+---------------+
| 2 IN('1,2,3') |
+---------------+
|             0 |
+---------------+
1 row in set, 1 warning (0.00 sec)

MariaDB [(none)]> SELECT FIND_IN_SET(2,'1,2,3');
+------------------------+
| FIND_IN_SET(2,'1,2,3') |
+------------------------+
|                      2 |
+------------------------+
1 row in set (0.00 sec)