Oracle count()函数

count(1)与count(*)比较：

如果你的数据表没有主键，那么count(1)比count(*)快 
如果有主键的话，那主键（联合主键）作为count的条件也比count(*)要快 
如果你的表只有一个字段的话那count(*)就是最快的啦 
count(*) count(1) 两者比较。主要还是要count(1)所相对应的数据字段。 
如果count(1)是聚索引,id,那肯定是count(1)快。但是差的很小的。 
因为count(*),自动会优化指定到那一个字段。所以没必要去count(?)，用count(*),sql会帮你完成优化的

count详解：

count(*)将返回表格中所有存在的行的总数包括值为null的行，然而count(列名)将返回表格中除去null以外的所有行的总数.
distinct+ 列名,得到的结果将是除去值为null和重复数据后的结果

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举例演示如下:

SQL> create table test
  2  (
  3  ename varchar2(10),
  4  sal number(4)
  5  );

表已创建。

SQL> insert into test values('fxe1',90);

已创建 1 行。

SQL> insert into test(ename) values('fxe2');

已创建 1 行。

SQL> insert into test(ename) values('fxe3');

已创建 1 行。

SQL> insert into test(ename) values('fxe4');

已创建 1 行。

SQL> insert into test values('fxe5',80);

已创建 1 行。

SQL> insert into test values('fxe6',80);

已创建 1 行。

SQL> select * from test;

ENAME             SAL                                                           
---------- ----------                                                           
fxe1               90                                                           
fxe2                                                                            
fxe3                                                                            
fxe4                                                                            
fxe5               80  

fxe6               80                                                        

SQL> select count(*) from test;

  COUNT(*)                                                                      
----------                                                                      
         6                                                                     

SQL> select count(sal) from test;

COUNT(SAL)                                                                      
----------                                                                      
         3                                                                     

SQL> select count(distinct sal) from test;

COUNT(DISTINCTSAL)                                                              
------------------                                                              
                 2                                                             

SQL> select distinct sal from test;

       SAL
----------
        80
        90