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;