mysql - 求助!如何创建一个新列来显示两个不同时期之间的持续时间

Question

在新数据集中，我创建了两个新列，Start_time 和 End_time。我还想创建一个列来显示这两个时间之间的差异。这是我的代码:

SELECT   b.company, 
     b.NAME, 
     Max( 
     CASE 
              WHEN description = 'Green' THEN final_value 
     END) AS 'Start_Time', 
     Max( 
     CASE 
              WHEN description = 'Red' THEN final_value 
     END) AS 'End_Time', 
     cast(datediff(n, 
       CASE('End_Time' as datetime), 
     CASE('Start_Time' AS datetime)) AS float) / 60 AS time_duration 
FROM     mydata a 
JOIN     ref_val b 
ON       a.ref_res = b.ref_res 
WHERE    b.company = 'abc' 
GROUP BY b.company , 
     b.NAME

当我运行这段代码时，我收到一条错误消息，指出无法绑定(bind) End_Time。有什么建议么？谢谢!

Answer 1

您可以使用子查询来使用您创建的别名，也不能使用 float 类型在 mysql 中转换值。

SELECT *, CAST(DATEDIFF(n, CAST(End_Time AS datetime), CAST(Start_Time AS datetime)) AS DECIMAL(10,2)) / 60 AS Time_Duration
  FROM (SELECT  B.COMPANY
               ,B.NAME
               ,MAX(case when DESCRIPTION = 'Green' then Final_Value end) as Start_Time
               ,MAX(case when DESCRIPTION = 'Red' then Final_Value end) as End_Time
          FROM mydata a
          JOIN ref_val B 
            ON A.ref_res = B.ref_res    
         WHERE B.COMPANY = 'abc'
         GROUP BY  B.COMPANY
                  ,B.NAME
        ) C