联表查询，查询右表有的数据或右表没有的数据

    SELECT
a.supvs_deptid as supvsDeptid,
       a.org_type as orgType,
       a.org_name as originalRegName,
       b.report_time as operateTime
   FROM
uac_org_main a
LEFT JOIN t_management_document_report b ON a.regcode = b.original_reg_code
where
a.status = 1
AND
a.valid_status = 1
AND
a.is_deleted = 'N'
       <if test="flag == true">
           a.regcode = b.original_reg_code
       </if>
       <if test="flag == false">
           and b.original_reg_code is NULL
       </if>