Question

我正在使用PostgreSQL数据库，并有一个表格如下：

--------------------------------------
| Date  | MetricType  | MetricValue  |
--------------------------------------
| date1 | MetricA     | val          |
--------------------------------------
| date1 | MetricB     | val          |
--------------------------------------
| date1 | MetricC     | val          |
--------------------------------------
| date2 | MetricA     | val          |
--------------------------------------
| date2 | MetricB     | val          |
--------------------------------------
| date2 | MetricC     | val          |
--------------------------------------

如您所见，每个日期都有一组度量标准类型，每种类型都有一个值。我想编写一个Select语句，它将这些数据组合在以下庄园中

------------------------------------------
| Date  | MetricA  | MetricB  | MetricC  | 
------------------------------------------
| date1 | val      | val      | val      |
------------------------------------------
| date2 | val      | val      | val      |
------------------------------------------

我不确定如何编写此Select语句以获得这些结果。有人可以帮忙吗？

Answer 1

请参阅http://www.artfulsoftware.com/infotree/queries.php#78

对于教程，您要查找的内容称为“数据透视”。这也可以使用CASE完成，如下所示：http：//stackoverflow.com/questions/1241178/mysql-rows-to-columns

Answer 2

此数据转换是 pivot 。如果您的数据库没有pivot函数，那么您可以使用带有CASE表达式的聚合函数：

select Date,
  max(case when MetricType = 'MetricA' then MetricValue end) MetricA,
  max(case when MetricType = 'MetricB' then MetricValue end) MetricB,
  max(case when MetricType = 'MetricC' then MetricValue end) MetricC
from yourtable
group by Date

请参阅SQL Fiddle with Demo

结果是：

|  DATE | METRICA | METRICB | METRICC |
---------------------------------------
| date1 |     val |     val |     val |
| date2 |     val |     val |     val |

您也可以使用表格上的多个联接来执行此操作：

select a.date as Date,
 a.MetricValue as MetricA,
 b.MetricValue as MetricB,
 c.MetricValue as MetricC 
from yourtable a
left join yourtable b 
  on a.date = b.date and b.MetricType = 'MetricB'
left join yourtable c 
  on a.date = c.date and c.MetricType = 'MetricC'
where a.MetricType = 'MetricA'

请参阅SQL Fiddle with Demo

Answer 3

实际上，Postgres 确实具有透视功能。安装附加模块tablefunc 在此相关问题下查找更多详细信息和解释：
PostgreSQL Crosstab Query

您的查询可能如下所示。假设指标属于数据类型integer：

SELECT * FROM crosstab(
   'SELECT date, metrictype, metricvalue 
    FROM   tbl
    ORDER  BY 1,2'  -- could also just be "ORDER BY 1" here

   ,$$VALUES ('MetricA'::text), ('MetricB'), ('MetricC')$$)
AS ct ("Section" text, "MetricA" int, "MetricB" int, "MetricC" int);

对于简单查询，CASE语句like demonstrated by @Bluefeet即可。但是这个查询可以更快地执行更多，并且对于更长的列/更大的表列表来说更短。

Answer 4

    select a.date as Date,
     a.metric as MetricType ,
b.metric as MetricType ,
     c.metric as MetricType 
    from (select distinct date from yourtable where metric = 'metricA')  a
    join yourtable b on a.date = b.date and b.metric = 'metricB'
    join yourtable c on a.date = c.date and c.metric = 'metricC'

SQL选择以使用Pivot或CASE组合输出表中的行值

4 个答案: