标签:full join 常见 问题 user SQL table id select
本文章持续记录工作中遇到的SQL的问题,持续更新中……
SQL常见问题
一、full join导致数据量翻倍
原因:空值会导致数据重复
错误SQL
select coalesce(a.user_id,b.user_id,c.user_id,d.user_id,e.user_id,f.user_id) as user_id
from
(select user_id from table_06)a full join
(select user_id from table_05)b on a.user_id=b.user_id full join
(select user_id from table_04)c on a.user_id=c.user_id full join
(select user_id from table_03)d on a.user_id=d.user_id full join
(select user_id from table_02)e on a.user_id=e.user_id full join
(select user_id from table_01)f on a.user_id=f.user_id
正确SQL
select coalesce(a.user_id,b.user_id,c.user_id,d.user_id,e.user_id,f.user_id) as user_id
from
(select user_id from table_06)a full join
(select user_id from table_05)b on a.user_id=b.user_id full join
(select user_id from table_04)c on coalesce(a.user_id,b.user_id)=c.user_id full join
(select user_id from table_03)d on coalesce(a.user_id,b.user_id,c.user_id)=d.user_id full join
(select user_id from table_02)e on coalesce(a.user_id,b.user_id,c.user_id,d.user_id)=e.user_id full join
(select user_id from table_01)f on coalesce(a.user_id,b.user_id,c.user_id,d.user_id,e.user_id)=f.user_id
二、left join 导致broadcast/mapjoin失效
原因:broadcast/mapjoin不经过reduce,读取文件后直接就会产生结果
小表有的key,left过程中不知道怎么处理。只能sortmergejoin
错误SQL
select count(1) from (
select count(1) from
(select pkg from trandw.dim_pub_app)a left join
(select gazj,pkg from trandw.dws_log_app_open_ds where dt='20220615' )b on a.pkg = b.pkg
)t ;
正确SQL
select count(1) from (
select count(1) from
(select pkg from trandw.dim_pub_app)a inner join
(select gazj,pkg from trandw.dws_log_app_open_ds where dt='20220615' )b on a.pkg = b.pkg
)t ;
标签:full,join,常见,问题,user,SQL,table,id,select 来源: https://www.cnblogs.com/wuxiaolong4/p/16426236.html
本站声明: 1. iCode9 技术分享网(下文简称本站)提供的所有内容,仅供技术学习、探讨和分享; 2. 关于本站的所有留言、评论、转载及引用,纯属内容发起人的个人观点,与本站观点和立场无关; 3. 关于本站的所有言论和文字,纯属内容发起人的个人观点,与本站观点和立场无关; 4. 本站文章均是网友提供,不完全保证技术分享内容的完整性、准确性、时效性、风险性和版权归属;如您发现该文章侵犯了您的权益,可联系我们第一时间进行删除; 5. 本站为非盈利性的个人网站,所有内容不会用来进行牟利,也不会利用任何形式的广告来间接获益,纯粹是为了广大技术爱好者提供技术内容和技术思想的分享性交流网站。