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sql1: select * from t1 where not exists (select a from t2 where t2.a = t1.a);

sql2: select * from t1 where t1.a not in (select a from t2 where t2.a is not null);

我认为sql1和sql2一样,他们会重写为anti join,对吧?

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1 回答 1

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如果您有NULL值,这两个查询将给出不同的结果。

Oracle 中的示例:

create table t1 (a integer);

insert into t1 (a) values (1);
insert into t1 (a) values (2);
insert into t1 (a) values (null);
commit;

create table t2 (a integer);

insert into t2 (a) values (1);
insert into t2 (a) values (3);
insert into t2 (a) values (null);
commit;

set null "{NULL}"

prompt First Query...

select * from t1 where not exists (select a from t2 where t2.a = t1.a);

prompt Second Query...

select * from t1 where t1.a not in (select a from t2 where t2.a is not null);

输出:

First Query...

         A
----------
         2
{NULL}    

2 rows selected.

Second Query...

         A
----------
         2

1 row selected.
于 2020-08-28T14:17:37.567 回答