Question

表有大约140万行。

1年查询：

SELECT windDir, 
count(windDir)/(
    SELECT COUNT(windDir) 
    FROM archive 
    WHERE windDir is not NULL AND dateTime >= 1466719200) as frequency 
FROM weewx.archive 
WHERE windDir is not NULL AND dateTime >= 1466719200 
GROUP BY windDir 
ORDER BY windDir ASC;

返回正确的结果（cba以复制频率结果）

+---------+-----------+
| windDir | frequency |
+---------+-----------+
|   0     |     x     |
|  22.5   |     x     |
|  45     |     x     |
|  67.5   |     x     |
|  90     |     x     |
| 112.5   |     x     |
| 135     |     x     |
| 157.5   |     x     |
| 180     |     x     |
| 202.5   |     x     |
| 225     |     x     |
| 247.5   |     x     |
| 270     |     x     |
| 292.5   |     x     |
| 315     |     x     |
| 337.5   |     x     |
+---------+-----------+

2年查询：

SELECT windDir, 
count(windDir)/(
    SELECT COUNT(windDir) 
    FROM archive 
    WHERE windDir IS NOT NULL AND dateTime >= 1435096800) as frequency 
FROM weewx.archive 
WHERE windDir IS NOT NULL AND dateTime >= 1435096800
GROUP BY windDir 
ORDER BY windDir ASC;

RETURNS WEIRD WRONG RESULTS

唯一的区别是dateTime，它会更改计算的行数。

这可能是某种内存问题吗？

甚至将查询简化为类似

的内容

SELECT DISTINCT windDir
from weewx.archive
WHERE dateTime >= 1435096800 
ORDER BY windDir ASC;

返回格式错误的查询，如上图所示。

这是怎么回事？

Answer 1

列winDir的类型为double，因此那些＆＃34;奇怪的格式错误＆＃34;结果只是存储在表中的double类型的值。（另请注意，它们的范围从0到360，因此它们是＆＃34;度＆＃34;。）如果您想要更容易阅读的更可预测的结果，那么可能通过使用调整winDir输出的精度像这样的ROUND（）：

select
       w.windir 
     , w.numerator / f.denominator AS frequency
from (
      SELECT
            ROUND(windDir,1) AS windDir
          , COUNT(windDir)   AS numerator
      FROM weewx.archive
      WHERE windDir IS NOT NULL
      AND DATETIME >= 1435096800
      GROUP BY
            ROUND(windDir,1)
     ) AS w
CROSS JOIN (
      SELECT
            COUNT(windDir) * 1.0 AS denominator
      FROM archive
      WHERE windDir IS NOT NULL
      AND DATETIME >= 1435096800
      ) AS f
ORDER BY windDir ASC
;

注意我已将分母的计算推送到交叉连接中，我认为它会提高效率。

查询太多行时，MySQL查询返回奇怪的格式错误的结果

1 个答案: