Question

以先前的问题为基础，此处（堆栈溢出）：How to efficiently pre-filter rows in conjunction with a join?

Nick为该问题提供了更为通用的解决方案，如下所示。

特别值得注意的是，每个“ diff / 2”第二次使用“ diff”，而我现在已经成功多次使用了该概念。

问题是我还有另一个无法正常运行的示例，我无法理解原因。似乎是MariaDB / mysql的“解析”问题，但我是新手。 ;-）

感谢您的见解。 = D

功能/工作示例：

SELECT scanid,
       event_id,
       object_id,
       obj_timestamp,
       obj_counter,
       diff,
       diff / 2 AS hdiff
FROM   (SELECT t1.scanid,
               t1.event_id,
               t1.object_id,
               t1.obj_timestamp,
               t1.obj_counter,
               Timediff(t1.obj_timestamp, t2.obj_timestamp) AS diff
        FROM   event_data AS t1
               LEFT JOIN event_data AS t2
                      ON t2.obj_counter = t1.obj_counter - 1
                         AND t2.object_id = t1.object_id
        WHERE  t1.object_id = 2
        ORDER  BY t1.obj_counter) AS diffs

我的代码-除非我尝试替换注释掉的显示行，否则此代码有效：

SET @distance=1000;
SELECT event_id,
       timestamp,
       t1.overall_time,
       TIME_TO_SEC(t1.overall_time) AS overall_secs,
--       @distance / overall_secs AS overall_kph
       @distance / TIME_TO_SEC(t1.overall_time) AS overall_kph
FROM   (SELECT event_id,
               timestamp,
               TIMEDIFF(MAX(timestamp), MIN(timestamp)) AS overall_time
        FROM   summary
        GROUP BY event_id) AS t1;

当我尝试将其指定为列后立即尝试参照total_secs时给出的错误（用注释的内容替换@distance行以查看此错误）：

ERROR 1054 (42S22) at line 120 in file: 'summary.sql': Unknown column 'overall_secs' in 'field list'

Answer 1

抱歉，SQL无法那样工作。直到多个步骤之后，“别名”才可用。我认为该标准允许SELECT中的“列”以任意顺序进行评估。

因此，您注释掉的行也可能是

@distance / TIME_TO_SEC(t1.overall_time) AS overall_kph

引用列时的MySQL意外范围问题

1 个答案: