SQL 运行很好，但如果用于创建视图，我得到 "Error Code: 1242. Subquery returns more than 1 row"

Question

我运行遇到了 MySQL 的一些奇怪问题。我有一个相当长的查询（在下面截断），其中包含子查询：

select
    SITEID as ID,
    "Predicted" as KIND,
    ATTRIBUTE as ATTRIBUTE_ID,
    "1" as TYPE,
    (select if(PREDICTEDRATE is null, 0, PREDICTEDRATE) from predictionperhour where siteid = ID and attribute = ATTRIBUTE_ID and TS = date_format((now() - interval 4 hour), '%Y-%m-%d %H:00:00')) as MINUS4,
    if(PREDICTEDRATE is null, 0, PREDICTEDRATE) as CURRENT,
    (select if(PREDICTEDRATE is null, 0, PREDICTEDRATE) from predictionperhour where siteid = ID and attribute = ATTRIBUTE_ID and TS = date_format((now() + interval 1 hour), '%Y-%m-%d %H:00:00')) as PLUS1,
from
    predictionperhour
where
    TS = date_format(now(), '%Y-%m-%d %H:00:00')
group by
    SITEID, 
    ATTRIBUTE

如果我运行此查询进入 SQL Workbench （不限制结果）我得到了我的 2027 行并且一切正常。现在，如果我使用完全相同的 select 创建一个视图，然后尝试一个简单的 select * 从那里我得到上面的错误 "Error Code: 1242. Subquery returns more than 1 row".

请问这是怎么发生的？我试图修改子查询来进行计数，但据我所知，所有结果都是 1 ... 我什至尝试添加一个 LIMIT 1，但尽管在这种情况下我没有遇到任何错误，但我说我没有从子查询中取回预期结果，而是取回了 0.

你能帮我找出问题出在哪里吗？

谢谢

Answer 1

像这样使用 tables (p0, p1, p2) 的别名。如果我们不给它取别名，那么可能会有列 attribute = ATTRIBUTE_ID 来自同一个 table predictionperhour AS p0 来自外部查询而不是来自内部查询。

select
    SITEID as ID,
    "Predicted" as KIND,
    ATTRIBUTE as ATTRIBUTE_ID,
    "1" as TYPE,
    (select if(PREDICTEDRATE is null, 0, PREDICTEDRATE) from predictionperhour AS p1 where p1.siteid = p0.ID and p1.attribute = p0.ATTRIBUTE_ID and TS = date_format((now() - interval 4 hour), '%Y-%m-%d %H:00:00')) as MINUS4,
    if(PREDICTEDRATE is null, 0, PREDICTEDRATE) as CURRENT,
    (select if(PREDICTEDRATE is null, 0, PREDICTEDRATE) from predictionperhour AS p2 where p2.siteid = p0.ID and p2.attribute = p0.ATTRIBUTE_ID and TS = date_format((now() + interval 1 hour), '%Y-%m-%d %H:00:00')) as PLUS1,
from
    predictionperhour AS p0
where
    TS = date_format(now(), '%Y-%m-%d %H:00:00')
group by
    SITEID, 
    ATTRIBUTE

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SQL 运行很好，但如果用于创建视图，我得到 "Error Code: 1242. Subquery returns more than 1 row"