oracle字符连接函数concat与||有什么区别

/*
CONCAT只能连接两个字符串
The syntax for the concat function is:
concat( string1, string2 )
string1 is the first string to concatenate.
string2 is the second string to concatenate.
*/
--||可以连接多个字符串
SQL> select concat('CSDN','_yeeXun') from dual;

CONCAT('CSDN','_YEEXUN')
------------------------
CSDN_yeeXun

SQL> select 'CSDN'||'_yeeXun' from dual;

'CSDN'||'_YEEXUN'
-----------------
CSDN_yeeXun

SQL> select concat('CSDN','_yeeXun','china') from dual;

select concat('CSDN','_yeeXun','china') from dual

ORA-00909: invalid number of arguments

SQL> select 'CSDN'||'_yeeXun'||'china' from dual;

'CSDN'||'_YEEXUN'||'CHINA'
--------------------------
CSDN_yeeXunchina--concat只能连接两个字符串，连接多个需要嵌套调用不方便SQL> select concat('aa','bb') from dual;  CONCAT('AA','BB')-----------------aabb --||直接连接多个字符串SQL> select 'aa'||'bb'||'cc' from dual;  'AA'||'BB'||'CC'----------------aabbcc  SQL> select concat(concat('aa',','),'bb') from dual这样得到的结果就是：CONCAT(CONCAT('AA',','),'BB')-----------------------------aa,bb