Oracle标量子查询

标量子查询

ORACLE允许在select子句中包含单行子查询, 使用标量子查询可以有效的改善性能，当使用到外部连接，或者使用到了聚合函数，就可以考虑标量子查询的可能性

1. 取消外部连接的使用

外部连接的做法：
select a.username,count(*)  from all_users a,all_objects b
where a.username=b.owner(+)
group by a.username;

 改成标量子查询的做法：
select a.username,(select count(*) from all_objects b where b.owner=a.username) cnt
from all_users a;

 PS: 两种做法得到的结果会有些许差别，主要在all_objects没有符合条件的行时, 外部连接的count(*)=1，而标量子查询的count(*)结果=0
select a.username,count(*),avg(object_id) from all_users a,all_objects b
where a.username=b.owner(+)
group by a.username; 

 2. 多个聚合函数的使用技巧

当同时出现count(*)/avg()时，不适合在select子句中调用两次子查询，性能上会受到影响, 可以改用下面两种做法

(1).拼接之后再拆分
select username,to_number(substr(data,1,10)) cnt,to_number(substr(data,11)) avg from
(
select a.username,(select to_char(count(*),'fm0000000009') || avg(object_id) from all_objects b where b.owner=a.username) data
from all_users a
)

 (2).创建对象类型
create or replace type myType as object
(cnt number,avg number);

select username,a.data.cnt,a.data.avg from
(
select username,(select myType(count(*),avg(object_id)) from all_objects b where b.owner=a.username) data
from all_users a
) a;