用户名表管理地区表，显示二级双地区信息

用户名表管理地区表，显示二级双地区信息

select f.add_id,(select a1.name from my.law_area a1 where a1.id = a.parent_id) as ,a.name from my.law_firm f left join  my.law_area a on f.area_id = a.id

select * from my.law_firm f left join  my.law_area a on f.area_id = a.id left join  my.law_area a1 on a.parent_id = a1.id

select 嵌套、关联查询优化

1. select .... from (subquery) as name ... ;

实例：
 select a1,a2,a3 
 from (select b1 as a1,b2 as a2,b3 as a3 from bbb) as aaa

 where a1 > 1;

 
2.用exists替代in嵌套查询：

实例：

  select a1 from aaa

  where a2 in(select b2 from bbb where b3 >10 );  效率很低

  select a1 from aaa

  where exists(select b2 from bbb where bbb.b1=aaa.a1);效率最高

 
 3.去掉字段中有'北京'的部分

update edb_bookweb_using.edb_hotelinfo

set hot_name_cn= trim(replace(hot_name_cn,'北京',''))

where  hot_name_cn like '%北京%';