计算字段后返回值不正确

Question

SELECT
    t3.id,
    t3.prod_ID,
    MIN(diff) AS min_time
FROM
    (SELECT
         t1.id,
         (UNIX_TIMESTAMP(t2.time_stamp_2) - UNIX_TIMESTAMP(t1.time_stamp)) AS diff
     FROM 
         production t1
     LEFT JOIN 
         process t2 ON t1.id = t2.id
     HAVING 
         diff >= 0) tx
LEFT JOIN 
    production t3 ON t3.id = tx.id
GROUP BY
    t3.id

运行后返回结果为：

id  prod_ID   min_time
-----------------------
1   2         1200

它应该返回的是

id  prod_ID   min_time
1   9         1200

我最初以为是加入有错误，经过多次加入测试，同样的错误结果。

SQLFiddle

SQLFiddle_2

SQLFiddle_3

SQLFiddle_2 澄清我使用 Group By 是因为我有多个 ID

SQLFiddle_3 进行更多扩展。

在SQLFiddle_3上运行后，返回结果为：

id  prod_ID   min_time
-----------------------
1   2         1200
2   2         960
3   2         360

应该是这样的

id  prod_ID   min_time
-----------------------
1   9         1200
2   2         960
3   3         360

Answer 1

在您的查询中，您仅GROUP BY t3.id，并且您不在t3.prod_ID 上进行聚合，因此返回的值是未定义的。

我相信你不需要GROUP BY，甚至不需要重新加入production。
试试这个：

SELECT t1.id, t1.prod_ID,
       (UNIX_TIMESTAMP(t2.time_stamp_2) - UNIX_TIMESTAMP(t1.time_stamp)) AS diff
FROM production t1 INNER JOIN process t2
ON t1.id = t2.id 
AND (t1.id, t1.prod_ID) = (
  SELECT p.id, p.prod_ID
  FROM production p
  WHERE p.id = t2.id AND (UNIX_TIMESTAMP(t2.time_stamp_2) - UNIX_TIMESTAMP(p.time_stamp)) >= 0
  ORDER BY (UNIX_TIMESTAMP(t2.time_stamp_2) - UNIX_TIMESTAMP(p.time_stamp)) LIMIT 1
)

请参阅demo。
结果：

> id | prod_ID | diff
> -: | ------: | ---:
>  1 |       9 | 1200
>  2 |       2 |  960

Answer 2

您的查询确实非常错误，您的内部查询本质上是列出时差，但是您将所有这些好的工作都扔掉了，并且不加入或分组 prod_id，所以它本质上只是选择第一个您插入数据库的值。如果您切换插入数据的顺序，因此先插入 prod_id，然后您会得到您想要的结果，但原因是错误的。

你的内部查询是这样的：

SELECT
    t1.id,
    t1.prod_id,
    (UNIX_TIMESTAMP(t2.time_stamp_2)-UNIX_TIMESTAMP(t1.time_stamp)) as diff
    FROM production t1
    left JOIN process t2
    ON t1.id = t2.id
   WHERE (UNIX_TIMESTAMP(t2.time_stamp_2)-UNIX_TIMESTAMP(t1.time_stamp)) > 0

如果你运行它，你会看到你想要的值：

id     prod_id    diff
1        2        1800
1        3        1380
1        9        1200

...但是您的外部查询对此没有任何作用。

有很多方法可以做到这一点，这里有一个简单的方法：

SELECT 
    t1.id,
    t1.prod_id,
    (UNIX_TIMESTAMP(t2.time_stamp_2)-UNIX_TIMESTAMP(t1.time_stamp)) as diff
    FROM production t1
    left JOIN process t2
    ON t1.id = t2.id
   WHERE (UNIX_TIMESTAMP(t2.time_stamp_2)-UNIX_TIMESTAMP(t1.time_stamp)) > 0
   ORDER BY 3
   LIMIT 1