Pivot a table with Amazon RedShift / PostgreSQL

问题:

I have several tables in Amazon RedShift that follow the pattern of several dimension columns and a pair of metric name/value columns.

DimensionA  DimensionB  MetricName  MetricValue ----------  ----------  ----------  ----------- dimA1       dimB1       m1          v11 dimA1       dimB2       m1          v12 dimA1       dimB2       m2          v21 dimA2       dimB2       m1          v13 dimA3       dimB1       m2          v22        

I am looking for a good way to unwind/pivot the data into a form of one row per each unique dimension set, e.g.:

DimensionA  DimensionB  m1   m2  ----------  ----------  ---  --- dimA1       dimB1       v11 dimA1       dimB2       v12  v21 dimA2       dimB2       v13 dimA3       dimB1            v22        

What is a good pattern for generating queries that would perform this unwinding?

Amazon RedShift is based on ParAccel and supports PostgreSQL 8.0.2, which does not have crosstab, unnest, pivot or unpivot.

回答1:

You can just create a CASE statement per MetricName but you'll have to use an aggregate as well to make the GROUP BY work.

SELECT dimension_a       ,dimension_b       ,MAX(CASE WHEN metric_name = 'm1' THEN metric_value ELSE NULL END) m1       ,MAX(CASE WHEN metric_name = 'm2' THEN metric_value ELSE NULL END) m2   FROM my_table  GROUP BY dimension_a          ,dimension_b ;

Worth noting that Redshift object names are never case sensitive but column content always is, which is the opposite of SQL Server defaults.

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