“不在”和“不存在”有什么区别？

Question

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);

我觉得sql1和sql2一样，都会改写成anti join吧？

Answer 1

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.