如何获得 mysql 列的计算输出？

Question

我需要使用 mysql 查询进行计算。

这是我的查询。

SELECT  book_name, 
CASE WHEN  CURDATE()<book_return THEN 0 ELSE DATEDIFF(CURDATE(),book_return) END AS DateDifference,
CASE WHEN  DateDifference>0 THEN DateDifference*10 ELSE NULL) END AS TotalFines  FROM tblIssuedBooks order by lastupdated DESC

如果 DateDIfference 值大于 zero.but，我需要将 DateDifference 列乘以 10，当我执行此操作时，我得到 Unknown column 'DateDifference' in 'field list' 作为错误。

有人可以告诉我如何改进吗？

Answer 1

您不能在 select 中重复使用同一个 select 中定义的别名。这里的一种解决方法是使用子查询：

SELECT book_name, DateDifference,
       CASE WHEN DateDifference > 0 THEN DateDifference*10 END AS TotalFines
FROM
(
    SELECT *, CASE WHEN CURDATE() < book_return
                   THEN 0
                   ELSE DATEDIFF(CURDATE(), book_return) END AS DateDifference
    FROM tblIssuedBooks
) t
ORDER BY lastupdated DESC;

如何获得 mysql 列的计算输出？

How to get a calculated output as mysql column?

mysql

sql