Absence of a value in a column of a row. Nulls indicate missing, unknown, or inapplicable data. 当一条记录的某个列为 NULL ，则表示这个列的值是未知的、是不确定的既然是未知的，就有无数种的可能性。 因此， NULL 并不是一个确定的值 -- 判断一个列为空 SELECT * FROM CUSTOMEREN C WHERE C.MODIFYDT IS NULL; -- 判断一个列非空 SELECT * FROM CUSTOMEREN C WHERE C.MODIFYDT IS NOT NULL; 测试： --NULL=NULL么： NULL is not equal to NULL SELECT 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么 ：空串 is not equal to NULL SELECT 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么： 空格 is not equal to NULL SELECT 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么: ‘‘ is NULL SELECT CASE WHEN ‘‘ IS NULL THEN ‘‘‘‘‘ is NULL‘ ELSE ‘‘‘‘‘ is not NULL‘ END "‘‘ is NULL?" FROM DUAL; --空串=空串么：‘‘ is not equal to ‘‘ SELECT CASE WHEN ‘‘ = ‘‘ THEN ‘‘‘‘‘ is equal to ‘‘‘‘‘ ELSE ‘‘‘‘‘ is not equal to ‘‘‘‘‘ END "‘‘‘‘=‘‘‘‘?" FROM DUAL; -- 将null替换成一个值 select 1 from dual where nvl(null,0)=nvl(null,0); NULL 排序问题 方法 1 将null替换成一个值 ,是用nvl 函数 select 1 from dual where nvl(null,0)=nvl(null,0); 方法2 是用NULLS LAST，NULLS FIRST处理 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
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