oracle 中运算符all注意

    oracle运算符all是关于集合的，如果集合为空，即取不到值，它与任何的<>、=等的比较，都返回真。奇怪....

    SQL> SELECT 1 result FROM dual WHERE 1<>1;
 
   RESULT
---------
 
SQL>

SQL> SELECT 1 RESULT
  2    FROM dual
  3   WHERE 1 <> ALL (SELECT 2 RESULT
  4            FROM dual
  5          UNION
  6          SELECT 3 RESULT FROM dual);
 
   RESULT
---------
        1
 
SQL>

SQL> SELECT 1 RESULT
  2    FROM dual
  3   WHERE 1 <> ALL (SELECT 2 RESULT FROM dual WHERE 1 <> 1);
 
   RESULT
---------
        1
 
SQL>