postgresql crosstab simple example - postgresql

Question

I got a key-value based table where each key-value pair is assigned to an entity which is identified by an id:
|_id__|_key_______|_value_|
| 123 | FIRSTNAME | John |
| 123 | LASTNAME | Doe |
And I want to transform it a structre like this:
|_id__|_firstName_|_lastName_|
| 123 | John | Doe |
I suppose one can use postgres build in crosstab function to do it.
Can you show me how to do it and explain why it works?

Answer 1

First of all activate the build in tablefunc-extension:
CREATE EXTENSION tablefunc;
Then create table and add sample data:
CREATE TABLE example (
id int,
key text,
value text
);
INSERT INTO example VALUES
(123, 'firstName', 'John'),
(123, 'lastName', 'Doe');
Now lets prepare the crosstab statment:
SELECT *
FROM example
ORDER BY id ASC, key ASC;
Its important to have the ORDER BY here.
Result:
|_id__|_key_______|_value_|
| 123 | FIRSTNAME | John |
| 123 | LASTNAME | Doe |
Solution
Now crosstab creates the table as we want:
SELECT *
FROM crosstab(
'SELECT *
FROM example
ORDER BY id ASC, key ASC;'
) AS ct(id INT, firstname TEXT, lastname TEXT);
Result:
|_id__|_firstName_|_lastName_|
| 123 | John | Doe |
How it works #1
To however understand how it works I found it easiest to just change the ORDER BY and see what happens:
SELECT *
FROM crosstab(
'SELECT *
FROM example
ORDER BY id ASC, key DESC;'
) AS ct(id INT, firstname TEXT, lastname TEXT);
Result:
|_id__|_firstName_|_lastName_|
| 123 | Doe | John |
As we changed the sorting of the key, the crosstab function sees the keys sorted in the other direction, thus reversing the generated columns.
How it works #2
Another thing that helped me understand how it works: the column definition is all about positions:
SELECT *
FROM crosstab(
'SELECT *
FROM example
ORDER BY id ASC, key ASC;'
) AS ct(blablafirst INT, blablasecond TEXT, blablathird TEXT);
Result
|_blablafirst__|_blablasecond_|_blablathird_|
| 123 | John | Doe |