SQL常见小问题
2022/7/1 2:21:29
本文主要是介绍SQL常见小问题,对大家解决编程问题具有一定的参考价值,需要的程序猿们随着小编来一起学习吧!
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 ;
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