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

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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

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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.