Oracle null 有哪些注意事项【面试题】

1 概述

null：空值，未指定的、未知的 或 不可预知的值。

2 示例

2.1 运算

select 1 + null 加,
       1 - null 减,
       1 * null 乘,
       1 / null 除
  from dual;
-- 结果都是 null

2.2 判断

with t_test as (
  select null a from dual union all
  select 1    a from dual
)
select t.*
  from t_test t
 where t.a is not null;
-- 返回 1

2.3 统计

with t_test as (
  select null a from dual union all
  select 1    a from dual union all
  select 1    a from dual
)
select count(1) a, -- 3
       count(*) b, -- 3
       count(t.a) c, -- 2
       count(distinct t.a) d -- 1
  from t_test t;

2.4 子查询

with t_test as (
  select null a from dual union all
  select 1    a from dual union all
  select 1    a from dual
)
select t.a
  from t_test t
 where t.a not in (null, 2);
-- 返回 null

2.5 连接

select 'a' || null a -- a
  from dual;

2.6 排序

• Oracle order by null 值排到最前面（nulls first）