Question

如果我使用MySQL case语句，它将创建一个新列，但不会替换该列。

是否可以替换列而不是新列？

示例：

SELECT *, 
       CASE WHEN housenumber_addition IS NULL THEN 'ABC' 
       ELSE housenumber_addition END AS housenumber_addition 
FROM customer

Answer 1

您可以有效地两次选择该列，就像您可以多次选择任何一列一样：

select housenumber_addition, housenumber_addition, housenumber_addition 
from customer

通常，您应该分别选择每一列，并用提供的代码替换housenumber_addition。 SELECT * is considered harmful anyways.

Answer 2

只需明确选择要显示在结果集中的列，就您而言，排除 housenumber_addition列即可：

SELECT
    col1,     -- some column, not housenumber_addition
    col2,     -- some other column, also not housenumber_addition
    ...,
    CASE WHEN housenumber_addition IS NULL
         THEN 'ABC' 
         ELSE housenumber_addition END AS housenumber_addition
FROM customer;

请注意，您的CASE表达式用途不大，因为您可能只使用过COALESCE：

COALESCE(housenumber_addition, 'ABC')

MySQL案例成为新专栏

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