MySQL - 将来自同一个表的多个选择组合成一个带有group by的结果表

Question

我有一个仪表读数表,其中包含一个米,没有读数值和读数日期.

我想显示每年每年读数的总和.

例如:

SELECT meterNo, SUM(readingValue) '2009' FROM readings
WHERE readingDate >= '2009-01-01' AND readingDate <= '2009-12-31'
GROUP BY meterNo;

将显示表格:

   meterNo  2009
   --------------
     4       50
     5       5
     12      30
     13      63
     18      18
     26      484
     27      21
     28      510
     29      28

这就是我想要的,它显示了2009年每个仪表的总读数值.但是,我需要结果表来显示每年的一列.所以它会是这样的:

   meterNo  2009  2010  2011  2012
   --------------------------------
     1             20    35     50
     2                   45     35
     3                   50     60
     4       50    35    20     30
     5       5     10    10     15
     6                          30

等等...

我可以重用查询的where部分:

WHERE readingDate >= '2009-01-01' AND readingDate <= '2009-12-31'

只需更改每年的日期,但如何在一个结果表中将每个WHERE结果显示为自己的列？

Answer 1

MySQL没有PIVOT函数,但您可以使用带CASE表达式的聚合函数将数据行转换为列.

如果您的数量有限years,那么您可以对查询进行硬编码:

select meterNo,
  sum(case when year(readingDate) = 2009 then readingValue else 0 end) `2009`,
  sum(case when year(readingDate) = 2010 then readingValue else 0 end) `2010`,
  sum(case when year(readingDate) = 2011 then readingValue else 0 end) `2011`,
  sum(case when year(readingDate) = 2012 then readingValue else 0 end) `2012`,
  sum(case when year(readingDate) = 2013 then readingValue else 0 end) `2013`
from readings
group by meterno;

请参阅SQL Fiddle with Demo

但是,如果要将未知数量的值或要作为新年度调整的查询添加到数据库中,那么可以使用预准备语句生成动态SQL:

SET @sql = NULL;
SELECT
  GROUP_CONCAT(DISTINCT
    CONCAT(
      'sum(CASE WHEN year(readingDate) = ',
      year(readingDate),
      ' THEN readingValue else 0 END) AS `',
      year(readingDate), '`'
    )
  ) INTO @sql
FROM readings;

SET @sql 
  = CONCAT('SELECT meterno, ', @sql, ' 
            from readings
            group by meterno');

PREPARE stmt FROM @sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;

请参阅SQL Fiddle with Demo.两者都给出了结果:

| METERNO | 2009 | 2010 | 2012 | 2013 | 2011 |
----------------------------------------------
|       1 |   90 |  180 |    0 |   90 |   90 |
|       2 |   50 |    0 |   90 |    0 |    0 |
|       3 |   80 |   40 |   90 |   90 |    0 |

作为旁注,如果要null在没有值而不是零的行中显示,则可以删除else 0(参见演示)