你现在是一名SQL工程师，请在我给出的代码前增加便于理解的注释。你需要处理的代码为： [SELECT dt,imei FROM (SELECT a.dt,a.imei,FIRST_VALUE(a.timestamp) OVER(PARTITION BY a.imei,a.dt ORDER BY TIMESTAMP ASC) first_time,a.timestamp,actiontype FROM hdp_kg_zf_splist.ods_dd_tb_app_action a WHERE a.dt between ‘${date1}’ and ‘${date2}’ AND from_unixtime(cast(TIMESTAMP/1000 AS int),’yyyyMMdd’)=a.dt]

-- 从ods_dd_tb_app_action表中选取日期在'${date1}'和'${date2}'之间，并且date与timestamp转换后的值与dt相等的记录  
SELECT dt,imei  
FROM  
(  
  -- 内嵌查询，按照imei、dt分组并按照时间戳升序排列，选取每组的第一个时间戳值作为first_time  
  SELECT a.dt,a.imei,FIRST_VALUE(a.timestamp) OVER(PARTITION BY a.imei,a.dt ORDER BY TIMESTAMP ASC) first_time,a.timestamp,actiontype  
  FROM hdp_kg_zf_splist.ods_dd_tb_app_action a  
  WHERE a.dt between '${date1}' and '${date2}' AND from_unixtime(cast(TIMESTAMP/1000 AS int),'yyyyMMdd')=a.dt  
)