MySQL CASE WHEN 混合 [compare_value] 和 [condition] 在 WHEN block 中不起作用

Question

Could someone please explain why this doesn't work as expected. And how to implement properly this logic on MySQL database? Example below:

create table test( effort text );
      insert into test values('0.1');
      insert into test values('0.25');
      insert into test values('1.5');
      insert into test values('5');
      insert into test values('10');

Sql

select effort, CASE effort 
    WHEN 0.1 THEN 5
    WHEN 0.25 THEN 4
    WHEN (CAST(effort AS DECIMAL(5,2)) < 3) THEN 3
    WHEN (CAST(effort AS DECIMAL(5,2)) between 3.0 AND 8.0) THEN 2
    WHEN (CAST(effort AS DECIMAL(5,2)) > 8) THEN 1
    END as 'ISE' from test

结果:

Answer 1

您不能混合使用两种类型的 case 语句——简单语句和搜索语句。因此，为每个条件添加条件:

select effort,
       (CASE WHEN effort = '0.1' THEN 5
             WHEN effort = '0.25' THEN 4
             WHEN CAST(effort AS DECIMAL(5,2)) < 3 THEN 3
             WHEN CAST(effort AS DECIMAL(5,2)) between 3.0 AND 8.0) THEN 2
             WHEN CAST(effort AS DECIMAL(5,2)) > 8 THEN 1
        END) as ISE
from test;

您会注意到我将类型切换为字符串比较的相等性。您正在将 effort 存储为字符串，因此比较应该作为字符串或使用显式比较。

我建议您将该值存储为 DECIMAL(5, 2) 而不是字符串。那么case会写成:

select effort,
       (CASE WHEN effort = 0.1 THEN 5
             WHEN effort = 0.25 THEN 4
             WHEN effort < 3 THEN 3
             WHEN effort between 3.0 AND 8.0) THEN 2
             WHEN effort > 8 THEN 1
        END) as ISE
from test;

我认为没有理由将该值存储为字符串，如果它是数字的话。