SQL - CASE WHEN 计算不同的值

Question

我需要显示每个 'id' 有多少不同的值。

应该是这样的:

id    |  component_a | component_b | component_c
--------------------------------------------------
KLS11 |     none     |      one    |     none       
KLS12 |     one      |      one    |     none         
KLS13 |     several  |      one    |     none        
KLS14 |     one      |      one    |     one            
KLS15 |     one      |    several  |     several           

我有下表(table_a):

id    |  component_a | component_b | component_c
--------------------------------------------------
KLS11 |              |      a      |            
KLS12 |       a      |      a      |              
KLS13 |       a      |      a      |             
KLS13 |       b      |      a      |               
KLS14 |       a      |      a      |      a        
KLS15 |       a      |      a      |      a                
KLS15 |       a      |      b      |      b

这里是一个例子/解释:

• KLS13 在 component_a ( a,b ) 中有不同的值 - 所以它应该显示 'several'
• KLS13 在 component_b ( a,a ) 中具有相同的值 - 所以它应该显示 '一个'
• KLS13 在 component_c 中没有值 - 所以它应该显示 'none'

这是我的 SQL 代码:

我已经为 component_a 做过，但它不起作用。我做错了什么？

SELECT 
CASE WHEN component_a is NULL THEN 'none'
     WHEN (SELECT count(DISTINCT component_a) 
             FROM table_a
              WHERE id=(SELECT id 
                          FROM table_a GROUP BY id HAVING count(*)>1)>1 THEN 'several'
     WHEN (SELECT count(DISTINCT component_a) 
             FROM table_a
              WHERE id=(SELECT id 
                          FROM table_a GROUP BY id HAVING count(*)>1)=1 THEN 'one'
END as componentA
FROM table_a

我是 SQL 的初学者，所以我将不胜感激。

祝你有美好的一天

Answer 1

您收到 ORA-00936 错误(我认为)是因为您没有关闭每个 when 分支中的括号；添加额外的关闭会将错误更改为“ORA-01427:单行子查询返回多行”，因为子子选择(带有 having 子句)返回多行 - 没有相关性。

您不需要子查询，只需将不同的值计算为 case 构造的一部分，即可创建 searched case expression :

select id,
  case count(distinct component_a)
    when 0 then 'none'
    when 1 then 'one'
    else 'several'
  end as component_a
from table_a
group by id
order by id;

ID    COMPONENT_A
----- -----------
KLS11 none        
KLS12 one         
KLS13 several     
KLS14 one         
KLS15 one         

然后重复其他列:

select id,
  case count(distinct component_a)
    when 0 then 'none'
    when 1 then 'one'
    else 'several'
  end as component_a,
  case count(distinct component_b)
    when 0 then 'none'
    when 1 then 'one'
    else 'several'
  end as component_b,
  case count(distinct component_c)
    when 0 then 'none'
    when 1 then 'one'
    else 'several'
  end as component_c
from table_a
group by id
order by id;

ID    COMPONENT_A COMPONENT_B COMPONENT_C
----- ----------- ----------- -----------
KLS11 none        one         none        
KLS12 one         one         none        
KLS13 several     one         none        
KLS14 one         one         one         
KLS15 one         several     several