确定Oracle null == null

Question

我希望在可为空的列上搜索数据库表。有时我要搜索的值本身是空的。由于Null等于nothing，甚至等于NULL，因此

where MYCOLUMN=SEARCHVALUE 

都会失败。现在我不得不求助于

where ((MYCOLUMN=SEARCHVALUE) OR (MYCOLUMN is NULL and SEARCHVALUE is NULL))

有没有更简单的说法呢？

(如果重要的话，我会使用Oracle )

Answer 1

你可以做IsNull或NVL之类的事情，但这只会让引擎做更多的工作。您将调用函数来执行列转换，然后必须对结果进行比较。

利用你所拥有的

where ((MYCOLUMN=SEARCHVALUE) OR (MYCOLUMN is NULL and SEARCHVALUE is NULL))

Answer 2

@Andy Lester断言，原始形式的查询比使用NVL更有效。我决定测试一下这个断言：

    SQL> DECLARE
      2    CURSOR B IS
      3       SELECT batch_id, equipment_id
      4         FROM batch;
      5    v_t1  NUMBER;
      6    v_t2  NUMBER;
      7    v_c1  NUMBER;
      8    v_c2  NUMBER;
      9    v_b   INTEGER;
     10  BEGIN
     11  -- Form 1 of the where clause
     12    v_t1 := dbms_utility.get_time;
     13    v_c1 := dbms_utility.get_cpu_time;
     14    FOR R IN B LOOP
     15       SELECT COUNT(*)
     16         INTO v_b
     17         FROM batch
     18        WHERE equipment_id = R.equipment_id OR (equipment_id IS NULL AND R.equipment_id IS NULL);
     19    END LOOP;
     20    v_t2 := dbms_utility.get_time;
     21    v_c2 := dbms_utility.get_cpu_time;
     22    dbms_output.put_line('For clause: WHERE equipment_id = R.equipment_id OR (equipment_id IS NULL AND R.equipment_id IS NULL)');
     23    dbms_output.put_line('CPU seconds used: '||(v_c2 - v_c1)/100);
     24    dbms_output.put_line('Elapsed time: '||(v_t2 - v_t1)/100);
     25  
     26  -- Form 2 of the where clause
     27    v_t1 := dbms_utility.get_time;
     28    v_c1 := dbms_utility.get_cpu_time;
     29    FOR R IN B LOOP
     30       SELECT COUNT(*)
     31         INTO v_b
     32         FROM batch
     33        WHERE NVL(equipment_id,'xxxx') = NVL(R.equipment_id,'xxxx');
     34    END LOOP;
     35    v_t2 := dbms_utility.get_time;
     36    v_c2 := dbms_utility.get_cpu_time;
     37    dbms_output.put_line('For clause: WHERE NVL(equipment_id,''xxxx'') = NVL(R.equipment_id,''xxxx'')');
     38    dbms_output.put_line('CPU seconds used: '||(v_c2 - v_c1)/100);
     39    dbms_output.put_line('Elapsed time: '||(v_t2 - v_t1)/100);
     40  END;
     41  /


    For clause: WHERE equipment_id = R.equipment_id OR (equipment_id IS NULL AND R.equipment_id IS NULL)
    CPU seconds used: 84.69
    Elapsed time: 84.8
    For clause: WHERE NVL(equipment_id,'xxxx') = NVL(R.equipment_id,'xxxx')
    CPU seconds used: 124
    Elapsed time: 124.01

    PL/SQL procedure successfully completed

    SQL> select count(*) from batch;

  COUNT(*)
----------
     20903

SQL> 

我有点惊讶地发现安迪是多么的正确。使用NVL解决方案的成本要高出近50%。因此，即使一段代码看起来不像另一段代码那样整洁或优雅，它的效率也会显著提高。我多次运行此过程，每次的结果几乎相同。向安迪致敬。

Answer 3

在Expert Oracle Database Architecture中，我看到：

WHERE DECODE(MYCOLUMN, SEARCHVALUE, 1) = 1