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NULL 是資料庫中特有的資料類型Oracle 中對空的描述
null
Absence of a value in a column of a row. Nulls indicate missing, unknown, or inapplicable data. 當一條記錄的某個列為 NULL ，則表示這個列的值是未知的、是不確定的既然是未知的，就有無數種的可能性。因此， NULL 並不是一個確定的值 -- 判斷一個列為空白SELECT * FROM CUSTOMEREN C WHERE C.MODIFYDT IS NULL;-- 判斷一個列非空SELECT * FROM CUSTOMEREN C WHERE C.MODIFYDT IS NOT NULL; 測試： --NULL=NULL麼： NULL is not equal to NULLSELECT CASE WHEN NULL = NULL THEN ‘NULL is equal to NULL‘ ELSE ‘NULL is not equal to NULL‘ END "NULL=NULL?" FROM DUAL; --空串=NULL麼 ：空串 is not equal to NULLSELECT CASE WHEN ‘‘ = NULL THEN ‘空串 is equal to NULL‘ ELSE ‘空串 is not equal to NULL‘ END "空串=NULL?" FROM DUAL; --空格=NULL麼： 空格 is not equal to NULLSELECT CASE WHEN ‘ ‘ = NULL THEN ‘空格 is equal to NULL‘ ELSE ‘空格 is not equal to NULL‘ END "空格=NULL?" FROM DUAL; --‘‘ 是NULL麼: ‘‘ is NULLSELECT CASE WHEN ‘‘ IS NULL THEN ‘‘‘‘‘ is NULL‘ ELSE ‘‘‘‘‘ is not NULL‘ END "‘‘ is NULL?" FROM DUAL; --空串=空串麼：‘‘ is not equal to ‘‘SELECT CASE WHEN ‘‘ = ‘‘ THEN ‘‘‘‘‘ is equal to ‘‘‘‘‘ ELSE ‘‘‘‘‘ is not equal to ‘‘‘‘‘ END "‘‘‘‘=‘‘‘‘?" FROM DUAL; -- 將null替換成一個值 select 1 from dual where nvl(null,0)=nvl(null,0); NULL 排序問題 方法 1 將null替換成一個值 ,是用nvl 函數 select 1 from dual where nvl(null,0)=nvl(null,0); 方法2 是用NULLS LAST，NULLS FIRST處理 SELECT c.modifydt FROM customeren c ORDER BY c.modifydt NULLS LAST;SELECT c.modifydt FROM customeren c ORDER BY c.modifydt NULLS FIRST;
【ORACLE】特殊的NULL
