oracle union 跟union all ,exists跟in

在sql 中据说使用，

exists 效率高于in,那么exists 跟in等价的sql如下,

aa跟bb表关联字段user_id

select * from aa where aa.user_id not in (select bb.user_id from bb);
select * from aa where not exists(select bb.user_id from bb where bb.user_id=aa.user_id);

select * from aa where aa.user_id in (select bb.user_id from bb);
select * from aa where exists(select bb.user_id from bb where bb.user_id=aa.user_id);

 

使用union 跟union all :

他们都可以关联多个表，不同的是：

union 会对查询的字段依据字段先后进行排序，去掉重复值。而union all 则不会去重复跟排序

SELECT USER_ID, USER_NAME FROM AA
UNION
SELECT USER_ID, USER_NAME FROM BB
;

SELECT USER_ID, USER_NAME FROM AA
UNION ALL
SELECT USER_ID, USER_NAME FROM BB
;