sql - PostgreSQL Crosstab Query

Question

Does any one know how to create crosstab queries in PostgreSQL?
For example I have the following table:

Section    Status    Count
A          Active    1
A          Inactive  2
B          Active    4
B          Inactive  5

I would like the query to return the following crosstab:

Section    Active    Inactive
A          1         2
B          4         5

Is this possible?

question from:https://stackoverflow.com/questions/65893831/using-sql-can-i-transpose-the-rows-into-columns

Answer 1

Install the additional module tablefunc once per database, which provides the function crosstab(). Since Postgres 9.1 you can use CREATE EXTENSION for that:

CREATE EXTENSION IF NOT EXISTS tablefunc;

Improved test case

CREATE TABLE tbl (
   section   text
 , status    text
 , ct        integer  -- "count" is a reserved word in standard SQL
);

INSERT INTO tbl VALUES 
  ('A', 'Active', 1), ('A', 'Inactive', 2)
, ('B', 'Active', 4), ('B', 'Inactive', 5)
                    , ('C', 'Inactive', 7);  -- ('C', 'Active') is missing

Simple form - not fit for missing attributes

crosstab(text) with 1 input parameter:

SELECT *
FROM   crosstab(
   'SELECT section, status, ct
    FROM   tbl
    ORDER  BY 1,2'  -- needs to be "ORDER BY 1,2" here
   ) AS ct ("Section" text, "Active" int, "Inactive" int);

Returns:

 Section | Active | Inactive
---------+--------+----------
 A       |      1 |        2
 B       |      4 |        5
 C       |      7 |           -- !!

• No need for casting and renaming.
• Note the incorrect result for C: the value 7 is filled in for the first column. Sometimes, this behavior is desirable, but not for this use case.
• The simple form is also limited to exactly three columns in the provided input query: row_name, category, value. There is no room for extra columns like in the 2-parameter alternative below.

Safe form

crosstab(text, text) with 2 input parameters:

SELECT *
FROM   crosstab(
   'SELECT section, status, ct
    FROM   tbl
    ORDER  BY 1,2'  -- could also just be "ORDER BY 1" here

  , $$VALUES ('Active'::text), ('Inactive')$$
   ) AS ct ("Section" text, "Active" int, "Inactive" int);

Returns:

 Section | Active | Inactive
---------+--------+----------
 A       |      1 |        2
 B       |      4 |        5
 C       |        |        7  -- !!

• Note the correct result for C.

• The second parameter can be any query that returns one row per attribute matching the order of the column definition at the end. Often you will want to query distinct attributes from the underlying table like this:

  'SELECT DISTINCT attribute FROM tbl ORDER BY 1'
  

  That's in the manual.

  Since you have to spell out all columns in a column definition list anyway (except for pre-defined crosstabN() variants), it is typically more efficient to provide a short list in a VALUES expression like demonstrated:

  $$VALUES ('Active'::text), ('Inactive')$$)
  

  Or (not in the manual):

  $$SELECT unnest('{Active,Inactive}'::text[])$$  -- short syntax for long lists
  

• I used dollar quoting to make quoting easier.

• You can even output columns with different data types with crosstab(text, text) - as long as the text representation of the value column is valid input for the target type. This way you might have attributes of different kind and output text, date, numeric etc. for respective attributes. There is a code example at the end of the chapter crosstab(text, text) in the manual.

db<>fiddle here

Advanced examples

• Pivot on Multiple Columns using Tablefunc - also demonstrating mentioned "extra columns"

• Dynamic alternative to pivot with CASE and GROUP BY

crosstabview in psql

Postgres 9.6 added this meta-command to its default interactive terminal psql. You can run the query you would use as first crosstab() parameter and feed it to crosstabview (immediately or in the next step). Like:

db=> SELECT section, status, ct FROM tbl crosstabview

Similar result as above, but it's a representation feature on the client side exclusively. Input rows are treated slightly differently, hence ORDER BY is not required. Details for crosstabview in the manual. There are more code examples at the bottom of that page.

Related answer on dba.SE by Daniel Vérité (the author of the psql feature):

• How do I generate a pivoted CROSS JOIN where the resulting table definition is unknown?

---

The previously accepted answer is outdated.

• The variant of the function crosstab(text, integer) is outdated. The second integer parameter is ignored. I quote the current manual:

  crosstab(text sql, int N) ...

  Obsolete version of crosstab(text). The parameter N is now ignored, since the number of value columns is always determined by the calling query

• Needless casting and renaming.

• It fails if a row does not have all attributes. See safe variant with two input parameters above to handle missing attributes properly.

• ORDER BY is required in the one-parameter form of crosstab(). The manual:

  In practice the SQL query should always specify ORDER BY 1,2 to ensure that the input rows are properly ordered