SELECT A.ID_ AS TASK_ID,
       A.PROC_INST_ID_ PROC_INST_ID,
       A.TASK_DEF_KEY_ AS ACT_ID,
       A.NAME_ AS ACT_NAME,
       A.ASSIGNEE_ AS ASSIGNEE,
       A.DELEGATION_ AS DELEGATION_ID,
       A.DESCRIPTION_ AS DESCRIPTION,
       A.CREATE_TIME_ as CREATE_TIME,
       A.DUE_DATE_ AS DUE_DATE,
       I.USER_ID CANDIDATE
  FROM ACT_RU_TASK A
  LEFT JOIN (SELECT DISTINCT * FROM (SELECT TASK_ID_, USER_ID_ as USER_ID
                    FROM ACT_RU_IDENTITYLINK I, ACT_RU_TASK T
                      WHERE TASK_ID_ IS NOT NULL
                        AND USER_ID_ IS NOT NULL
                        AND I.TASK_ID_ = T.ID_
                        AND T.ASSIGNEE_ IS NULL
                        AND TYPE_ = 'candidate'
                     UNION
                     SELECT TASK_ID_,USER_ID_ as USER_ID
                       FROM ACT_RU_IDENTITYLINK I,ACT_RU_TASK T
                      WHERE I.TASK_ID_ IS NOT NULL
                        AND I.GROUP_ID_ IS NOT NULL
                        AND I.TASK_ID_ = T.ID_
                        AND T.ASSIGNEE_ IS NULL
                        AND TYPE_ = 'candidate')U) I
    ON A.ID_ = I.TASK_ID_