优化数据更新

今天写了一条数据更新操作的SQL语句
如下
update shop as t1
set
t1.STREET=
(select  distinct (concat(t2.DSCP,"  ",coalesce(t2.STREET)))
   from mall as t2
   where t1.MALL_CD=t2.CODE),
t1.STREET_LOC_1=
   (select  distinct (concat(t2.DSCP_LOC_1,"  ",coalesce(t2.STREET_LOC_1)))
  from mall as t2
  where t1.MALL_CD=t2.CODE),
t1.STREET_LOC_2=
  (select  distinct (concat(t2.DSCP_LOC_2,"  ",coalesce(t2.STREET_LOC_2)))
  from mall as t2
  where t1.MALL_CD=t2.CODE);
当时也没考虑很多【数据量比较大】，结果把语句执行好才意识到，效率太低了，一位前辈告诉我其实那个更新语句可以如下所示，效率会提高很多。
update shop as sh, mall as ma set sh.STREET=concat(ma.DSCP,' ',ma.STREET),sh.STREET_LOC_1=concat(ma.DSCP_LOC_1,' ',ma.STREET_LOC_1),sh.STREET_LOC_2=concat(ma.DSCP_LOC_2,' ',ma.STREET_LOC_2) where sh.MALL_CD=ma.CODE
对照比较发现，的确如此。看来以后要注意了！