为什么删除有主键的表的时候，条件不能是非主键字段

代码如下(plsql)：

SQL> desc tb27_1

Name  Type      Nullable Default Comments 

----- --------- -------- ------- -------- 

ID    NUMBER(3)                           

GARDE NUMBER(3) Y     

//id是主键

SQL> select * from tb27_1  ;

  ID GARDE

---- -----

   1     2

   3     4

SQL> delete from tb27_1 where garde=2;

delete from tb27_1 where GRADE=2

ORA-00904: "GRADE": 标识符无效

SQL> delete from tb27_1 where id=1;

1 row deleted