رفتار NULL در شرط های اوراکل

رفتار NULL  را در عملگرهای مختلف را در Oracle مشاهده می کنیم:

مثال اول: همانطور که مشاهده می کنید NULL = NULL در اوراکل هیچ رکوردی بر نمیگردد.

select '1=0' as test from dual where 1 = 0
Union
select '1=1' as test from dual where 1 = 1
Union
select '1=NULL' as test from dual where 1 = NULL
Union
select 'NULL=NULL' as test from dual where NULL = NULL


Result:
1=1

مثال دوم: همانطور که مشاهده می کنید NULL != NULL در اوراکل هیچ رکوردی بر نمیگردد.

select '1!=1' as test from dual where 1!= 1
Union
select '1!=NULL' as test from dual where 1 != NULL
Union
select 'NULL=NULL' as test from dual where NULL != NULL

Result:
---

مثال سوم: همانطور که مشاهده می کنید  هر موقع از IS در اوراکل برای NULL استفاده کنیم رکورد بر میگرداند.

select '1 IS NULL' as test from dual where 1 IS NULL
Union
select 'NULL IS NULL' as test from dual where NULL IS NULL
Union
select 'NULL IS NOT NULL' as test from dual where NULL IS NOT NULL
Union
select '1 IS NOT NULL' as test from dual where 1 IS NOT NULL

Result:
1 IS NOT NULL
NULL IS NULL

مثال چهارم:

select 'True and NULL' as test from dual where ((1=1) and (NULL=NULL))
Union
select 'False and NULL' as test from dual where ((1=2) and (NULL=NULL))
Union
select 'True or NULL' as test from dual where ((1=1) or (NULL=NULL))
Union
select 'False and NULL' as test from dual where ((1=2) or (NULL=NULL))

Result:
True or NULL