當混合[compare_value]和[condition]在WHEN塊中的條件不起作用時，MySQL CASE

Question

有人可以解釋為什么這不能按預期進行。 以及如何在MySQL數據庫上正確實現此邏輯？ 下面的例子：

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

結果：

Answer 1

您不能混合使用兩種類型的case語句-簡單語句和搜索語句。 因此，為每個條件設置條件：

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;

您會注意到，我為字符串比較切換了相等類型。 您將effort存儲為字符串，因此比較應為字符串或使用顯式比較。

相反，我建議您將值存儲為DECIMAL(5, 2)而不是字符串。 然后，該case將寫為：

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;

我認為沒有必要將值存儲為字符串（如果打算將其用作數字）。

Answer 2

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;