mysql - 使用变量并重用它时，SELECT 语句无法正确显示数据

Question

我有一个 mysql 查询，它将结果存储在变量中，但是当我稍后使用相同的变量时，它不会返回相同的值。我错过了什么吗？

SELECT 
    duration,
    @sum_duration := (SUM(duration) OVER (
        PARTITION BY user_id,`Date`
    )) as sum_duration,
    @sum_duration
    -- 9*3600*1000*(duration/@sum_duration) as normalized_duration
FROM duration_table

上述查询的结果是

'duration', 'sum_duration', '@sum_duration'
'86400000', '172800000',    '783413.000000000000000000000000000000'
'86400000', '172800000',    '783413.000000000000000000000000000000'
'86400000', '172800000',    '783413.000000000000000000000000000000'
'86400000', '172800000',    '172800000.000000000000000000000000000000'
'86400000', '172800000',    '172800000.000000000000000000000000000000'
'86400000', '172800000',    '172800000.000000000000000000000000000000'
'86400000', '172800000',    '172800000.000000000000000000000000000000'
'86400000', '172800000',    '172800000.000000000000000000000000000000'
'86400000', '172800000',    '172800000.000000000000000000000000000000'
'86400000', '172800000',    '172800000.000000000000000000000000000000'

Answer 1

这个查询看起来不错:

SELECT 
    duration,
    @sum_duration := (SUM(duration) OVER (PARTITION BY user_id,`Date`))
FROM duration

不带order by的窗口总和为您提供相同user_id和date的总持续时间。

但是我在这里看不到变量的意义。我注意到您的查询中的注释行:

SELECT
    duration
    @sum_duration := (SUM(duration) OVER (PARTITION BY user_id,`Date`)),
    9*3600*1000*(duration/@sum_duration) as normalized_duration --> here
FROM duration

但是，这可能无法按预期工作，因为 MySQL 不保证 SELECT 子句中表达式的计算顺序。

也许，您想要:

SELECT 
    duration,
    SUM(duration) OVER (PARTITION BY user_id,`Date`) sum_duration,
    9 * 3600 * 1000 * duration 
        / (SUM(duration) OVER (PARTITION BY user_id,`Date`)) normalized_duration    
FROM duration

您还可以使用嵌套子查询来表达这一点:

SELECT
    t.*,
    9 * 3600 * 1000 * duration / sum_duration normalized_duration
FROM (
    SELECT 
        duration,
        SUM(duration) OVER (PARTITION BY user_id,`Date`) sum_duration    
    FROM duration
) t