Order by n...是什么意思？

For Example:
 
SQL>   select   *   from   test_tab   order   by   1;  
   
  COL_A                       COL_B  
  ----------                ----------  
  A                              1  
  A                              2  
  B                              2  
  B                              3  
  C                              4  
  D                              1  
  D                              12  
  D                              121  
  D                              12  
   
 
  9   rows   selected  
 
===============================================
 
  SQL>   select   *   from   test_tab   order   by   2;  
 
  COL_A                       COL_B  
  ----------                 ----------  
  A                              1  
  D                              1  
  A                              2  
  B                              2  
  B                              3  
  C                              4  
  D                              12  
  D                              12  
  D                              121  
   
 
  9   rows   selected  
 
===============================================
  SQL>   select   *   from   test_tab   order   by   3;  
   
  select   *   from   test_tab   order   by   3  
   
 
  ORA-01785:   ORDER   BY   项必须是   SELECT-list   表达式的数目  
 
===============================================
   
   
  以上看来:1表示第一个栏位, 2表示第二栏位, 依此类推  
  当表中只有2个栏位时, ORDER  BY  3 就会出错

转载自：https://blog.csdn.net/seaee/article/details/47253475