MYSQL中的相关子查询

Question

任何人都可以帮助我，我有相关的嵌套子查询的问题，即，当我试图在嵌套的子查询中使用祖父列时，我得到了这个错误

错误代码：1054'where子句'中的未知列'scu.iUserId'

查询：

SELECT 
  scu.iUserId,
  (SELECT 
    SUM(
      sbs.`iNoPointsBeginning` + 
      (SELECT 
        COALESCE(SUM(BehaviorPts), 0) AS StudentPts 
      FROM
        (SELECT 
          (
            COUNT(sbis.iIncidentSubmissionId) * sbi.iPointValue
          ) AS BehaviorPts 
        FROM
          scn_behavior_incident_submission sbis 
          JOIN scn_behavior_incident_actors sbia 
            ON sbia.iIncidentSubmissionId = sbis.iIncidentSubmissionId 
          LEFT JOIN scn_behavior_incidents sbi 
            ON sbi.iIncidentId = sbis.iBehaviorIncidentId 
        WHERE sbia.iUserId = scu.iUserId
          AND sbia.eActorType  'Witness' 
          AND sbis.iSchoolId = '97' 
        GROUP BY sbis.iBehaviorIncidentId) AS BehaviorTotal)
    ) AS stu_pt 
  FROM
    scn_behavior_settings sbs 
  WHERE sbs.`iSchoolId` = '97') 
FROM
  scn_sections_members AS scm 
  INNER JOIN scn_users AS scu 
    ON scu.iUserId = scm.iStudentId