How Can I Dynamically Transform Row-Oriented Data into Column-Oriented Data in PostgreSQL?

Alternatives for dynamically converting row data to column data in PostgreSQL: CASE and GROUP BY vs. crosstab function

In PostgreSQL, many users need to convert row data tables into column data tables for analysis and reporting. While CASE and GROUP BY are commonly used methods, more dynamic and efficient alternatives exist.

Problem Statement

For example, consider the following table:

id	feh	bar
1	10	A
2	20	A
3	3	B
4	4	B
5	5	C
6	6	D
7	7	D
8	8	D

Desired Output Convert this data to the following format:

bar	val1	val2	val3
A	10	20	NULL
B	3	4	NULL
C	5	NULL	NULL
D	6	7	8

Solution using CASE and GROUP BY

One way to achieve this is to use the CASE and GROUP BY techniques:

SELECT bar, 
   MAX(CASE WHEN abc."row" = 1 THEN feh ELSE NULL END) AS "val1",
   MAX(CASE WHEN abc."row" = 2 THEN feh ELSE NULL END) AS "val2",
   MAX(CASE WHEN abc."row" = 3 THEN feh ELSE NULL END) AS "val3"
FROM
(
  SELECT bar, feh, row_number() OVER (partition by bar) as row
  FROM "Foo"
 ) abc
GROUP BY bar 

While this approach works, it has potential limitations in terms of flexibility and performance.

crosstab Functions: a dynamic solution

The

function introduced by the tablefunc module in PostgreSQL is a more dynamic and efficient alternative. Using the crosstab() function, you can achieve the desired transformation without explicitly defining the output columns. crosstab()

Basic

Solutioncrosstab

For the example problem, the basic

solution is as follows: crosstab

SELECT * FROM crosstab(
  'SELECT bar, 1 AS cat, feh
   FROM   tbl_org
   ORDER  BY bar, feh')
 AS ct (bar text, val1 int, val2 int, val3 int);

In this query,

provides data in a format compatible with SELECT bar, 1 AS cat, feh. The crosstab() column is used as a dummy placeholder since there is no category information in the table. cat

Advanced

Solutionscrosstab

If an actual category column is present, you can use it to control the order of the values ​​in the results. The following query demonstrates this:

SELECT * FROM crosstab(
   $$
   SELECT bar, val, feh
   FROM  (
      SELECT *, 'val' || row_number() OVER (PARTITION BY bar ORDER BY feh) AS val
      FROM tbl_org
      ) x
   ORDER BY 1, 2
   $$
 , $$VALUES ('val1'), ('val2'), ('val3')$$         
) AS ct (bar text, val1 int, val2 int, val3 int);

News

crosstab

While the above solution provides dynamic column generation, dynamically returning different columns in PostgreSQL is more challenging. However, under certain restrictions, this can be achieved using the built-in functionality of the

module. tablefunc

Conclusion

The

function provides a powerful and dynamic alternative to the crosstab() and CASE methods. It allows flexible data transformation, making it a valuable tool for data manipulation in PostgreSQL. GROUP BY

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