group_concat

SELECT 
  t.application_id,
  t.submit_by,
  t.dept_code,
  t.create_time,
  t.complete_time,
  t.`process_id`,
  t_hi_new.assignee
FROM
  t_application_info t 
  LEFT JOIN 
    (SELECT 
      t_hi.`PROC_INST_ID_` AS process_id,
      GROUP_CONCAT(CONCAT(t_hi.`ASSIGNEE_`)) AS assignee 
    FROM
      act_hi_taskinst t_hi 
    GROUP BY PROC_INST_ID_) t_hi_new 
    ON t.`process_id` = t_hi_new.process_id 
WHERE t.`application_type` IN ('50603') 
  AND UNIX_TIMESTAMP(t.create_time) > UNIX_TIMESTAMP('2019-01-01 00:00:00') 
  AND UNIX_TIMESTAMP(t.complete_time) < UNIX_TIMESTAMP('2019-12-31 23:59:59') ;