sql - SQL Oracle：用单词替换空结果

Question

我正在研究这个问题好几天。我有一个oracle数据库。问题必须在一个查询中解决。No Function, Pocedure, ... 我要进行选择。当他有结果时，发布它们。否则应该有“空结果”。

select case 
when count(*) = 0    
then 'no Entry'   
else MAX(Member)--all Members should be here 
END as Member 
from tableMember 
where Membergroup = 'testgroup'; 

问题是 Oracle 想要一个由 else 提供的 Agregat 函数。因此，如果结果不是“无条目”，我只会得到一个值。我需要所有的价值观。

欢迎所有能帮助我的人，让我快乐。

Answer 1

不知道你想达到什么目标，也许这个

 select member from tablemember where Membergroup = 'testgroup'
union 
 select  'no Entry' 
 from dual 
 where NOT EXISTS ( select member from tablemember where membergroup = 'testgroup')
;

Answer 2

不需要两个聚合查询，你只需要检查是否max(member)为空。我会这样做是为了弄清楚发生了什么。

select case when max_member is null then 'no entry' else max_member end as member
  from ( select max(member) as max_member
           from tablemember
          where membergroup = 'testgroup'
                )

但是，如果您想返回所有成员，您可以执行以下操作：

select member
  from tablemember
 where membergroup = 'testgroup'
 union all
select 'no entry'
  from dual
 where not exists ( select 1 from tablemember where membergroup = 'testgroup')

Answer 3

如果您RIGHT JOIN使用空集的查询进行查询，您将始终获得一行，并且如果您的查询返回数据，则将获得所有行。这比 aUNION或UNION ALL使用 a更便宜（更快），NOT EXISTS因为它不需要对数据进行多次扫描。

SELECT nvl(a.member,b.member) member
   FROM (SELECT member FROM tablemember WHERE membergroup='????') a
   RIGHT JOIN (SELECT 'no Entry' member FROM dual) b ON 1=1;

测试环境：

DROP TABLE tablemember;
CREATE TABLE tablemember AS
   (
      SELECT TO_CHAR(level) member
         , DECODE(mod(level, 5), 0, 'testgroup', 'othergroup') membergroup
      FROM dual CONNECT BY level <= 50
   );

Answer 4

您可以使用一些聚合函数和 NVL 来实现您的目标：

SELECT MIN('VALUE 1') AS p1, MIN('VALUE 2') AS p2 FROM DUAL WHERE 1=0

此查询的结果是：NULL，NULL

接下来，用所需的字符串替换空值：

SELECT 
    NVL(MIN('1'), 'empty  value 1') AS p1, 
    NVL(MIN('STRING VALUE'), 'empty value 2')  AS p2,
    NVL(MIN((select 'subquery result' from dual)), 'empty subquery result') as p3
FROM 
    DUAL 
WHERE 
    1=0

但是，您不能在字段中混合数字和字符串。

Answer 5

试试这个：

DECLARE C INTEGER;

SELECT COUNT(*) INTO C FROM tableMember WHERE Membergroup = 'testgroup';

IF C > 0

THEN

    SELECT * FROM tableMember;

ELSE

    SELECT 'No results!' FROM tableMember;

END IF;