Not in 改写左连接不需要关注连接列是否重复数据

SQL> select * from a1;        ID NAME---------- ----------         1 a         1 a         1 a         2 a         2 a         2 a         3 a         3 a         3 a已选择9行。SQL> select * from a2;        ID NAME---------- ----------         2 a         2 a         2 a         3 a         3 a         3 a已选择6行。not in :SQL> select * from a1 where id not in (select id from a2);        ID NAME---------- ----------         1 a         1 a         1 a左连接:SQL> select * from a1 left join a2 on a1.id=a2.id  2  where a2.id is null;        ID NAME               ID NAME---------- ---------- ---------- ----------         1 a         1 a         1 a