('xyz‘!= 'xyz')和不是('xyz’= 'xyz')有什么区别？

Question

('xyz' != 'xyz')和NOT ('xyz' = 'xyz')有什么区别吗？

虽然我询问的是Oracle DB的上下文，但也希望了解其他语言中的相同情况。

不同的主要是处理速度和所有的背景资料。

Answer 1

你觉得有什么不同吗？

测试用例：

SQL> SELECT * FROM dual
  2  WHERE ('xyz' != 'xyz');

no rows selected

SQL>
SQL> SELECT * FROM dual
  2  WHERE NOT ('xyz' = 'xyz');

no rows selected

在上述两个查询中，应用的过滤器都是相同的过滤器(NULL不是NULL)。

执行计划：

SQL> EXPLAIN PLAN FOR
  2  SELECT * FROM dual
  3  WHERE ('xyz' != 'xyz');

Explained.

SQL>
SQL> SELECT * FROM TABLE(dbms_xplan.display);

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Plan hash value: 3752461848

---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     1 |     2 |     0   (0)|          |
|*  1 |  FILTER            |      |       |       |            |          |
|   2 |   TABLE ACCESS FULL| DUAL |     1 |     2 |     2   (0)| 00:00:01 |
---------------------------------------------------------------------------

Predicate Information (identified by operation id):

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
---------------------------------------------------

   1 - filter(NULL IS NOT NULL)

14 rows selected.

SQL>
SQL> EXPLAIN PLAN FOR
  2  SELECT * FROM dual
  3  WHERE NOT ('xyz' = 'xyz');

Explained.

SQL>
SQL> SELECT * FROM TABLE(dbms_xplan.display);

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Plan hash value: 3752461848

---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     1 |     2 |     0   (0)|          |
|*  1 |  FILTER            |      |       |       |            |          |
|   2 |   TABLE ACCESS FULL| DUAL |     1 |     2 |     2   (0)| 00:00:01 |
---------------------------------------------------------------------------

Predicate Information (identified by operation id):

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
---------------------------------------------------

   1 - filter(NULL IS NOT NULL)

14 rows selected.

SQL>

没什么区别。