in/exists not in/not exists null

in/not in exists/not exists null的理解

两个测试表

create table tmp01 as

with tmp as (

select '1' as id from dual

union all

select '2' as id from dual

union all

select '3' as id from dual

union all

select null as id from dual

)

select * from tmp;

create table tmp02 as

with tmp as (

select '1' as id from dual

union all

select '2' as id from dual

union all

select null as id from dual

)

select * from tmp;

select * from tmp01 t where t.id in (select * from tmp02);

ID

in可以理解为 t.id = 1 or t.id = 2 or t.id = null

select * from tmp01 t where t.id not in (select * from tmp02);

ID

no rows

not in 可以理解为 t.id <> 1 and t.id <> 2 and t.id <> null

由于t.id <> null,为unkown,始终返回false,所以查不出值来。

解决：

select * from tmp01 t where t.id not in (select * from tmp02 where id is not null) or t.id is null;

---------------------------------------------------------------------------------------------------------------------------------

exists实际上用的也是等值判断

select * from tmp01 t where exists (select 'X' from tmp02 d where d.id = t.id);

ID

子查询中，d.id = t.id 判断出来的只有  1 ，2，null = null不可判断，null只能使用 is null,is not null.

select * from tmp01 t where not exists (select 'X' from tmp02 d where d.id = t.id);

ID

此语句查出了null和3，子查询中查出的是1,2，不在这个集合中的有null和3

再说一下 in/exists的效率问题

两个表中一个数据量小，一个数据量大，则子查询表大的用exists，表小的用in

表tmp01（小表），表tmp02（大表）

select * from tmp01 where id in (select id from tmp02)

效率低，用到了tmp01 表上id 列的索引

select * from tmp01 where exists(select id from tmp02 where id= tmp01.id)

效率高，用到了tmp02 表上id 列的索引。

select * from tmp02 where exists(select id from tmp01 where id= tmp02.cc)

效率高，使用tmp02 表上id 列的索引

select * from tmp02 where exists(select id from tmp01 where id= tmp02.cc)

效率低，用到了tmp01表上id列的索引

not in/not exists

not in内外表进行全表扫描，不使用索引

not extsts 的子查询用到表上的索引。无论表大小，用not exists 都比not in 要快。