/sgivee/article/details/5081350SQL中cross join，left join，right join ，full join，inner join 的区别cross join是笛卡儿乘积 就是一张表的行数乘以另一张表的行数
left join第一张表的连接列在第二张表中没有匹配是,第二张表中的值返回null
right join第二张表的连接列在第一张表中没有匹配是,第一张表中的值返回null
full join返回两张表中的行 left join+right join
inner join只返回两张表连接列的匹配项 /blog/559014 insertintot_sales_forecast selectSALES_FORECAST_SEQUENCE.nextvalasid,basic.*from ( selectschool_id,district_id,year_table.year,month_table.month,500asor_forcast,1000asre_forcast,sysdateasupdate_date,'1'asuser_id,sysdateasinsert_date fromv_school_district crossjoin ( selectlevelasyearfromdualwherelevel>connectbylevel< )year_table crossjoin ( selectlpad(level,2,0)monthfromdualconnectbylevel<13 )month_table orderbyschool_id,district_id,year_table.year,month_table.month )basic
