I am using a PostgreSQL database and have a table as follows:
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| Date | MetricType | MetricValue |
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| date1 | MetricA | val |
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| date1 | MetricB | val |
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| date1 | MetricC | val |
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| date2 | MetricA | val |
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| date2 | MetricB | val |
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| date2 | MetricC | val |
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As you can see, each date has a set of metric types and each type has a value. I want to write a Select statement which combines this data in the following manor
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| Date | MetricA | MetricB | MetricC |
------------------------------------------
| date1 | val | val | val |
------------------------------------------
| date2 | val | val | val |
------------------------------------------
I am not sure how to go about writing this Select statement to get these results. Can anyone help?
See http://www.artfulsoftware.com/infotree/queries.php#78
For a tutorial, what you are looking for is called a "pivot" This can also be done using CASE as shown here:http://stackoverflow.com/questions/1241178/mysql-rows-to-columns
This data transformation is a pivot . If your database doesn't have a pivot function, then you can use an aggregate function with a CASE
expression:
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
The result is:
| DATE | METRICA | METRICB | METRICC |
---------------------------------------
| date1 | val | val | val |
| date2 | val | val | val |
You can also do this using multiple joins on the table:
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'
Actually, Postgres does have pivot functions. Install the additional module tablefunc .
Find more details and explanation under this related question:
PostgreSQL Crosstab Query
Your query could look like this. Assuming the metrics to be of data type 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);
For simple queries a CASE
statement like demonstrated by @Bluefeet will do. But this query performs much faster and is much shorter for longer lists of columns / bigger tables.
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'
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