Transform JSON array to boolean columns in PostgreSQL - arrays

Question

I have a column that contains a JSON array of strings, which I would like to transform into boolean columns. These columns are true if the value was present in the array.
Let's say I have the following columns in Postgres.
|"countries"|
---------------
["NL", "BE"]
["UK"]
I would like to transform this into boolean columns per market. e.g.
|"BE"|"NL"|"UK"|
--------------------
|True|True|False|
|False|False|True|
I know I can manually expand it using case statements for each country code, but there are 200+ countries.
Is there are more elegant solution?

Answer 1

Displaying a various list of columns whose labels are known only at the runtime is not so obvious with postgres. You need some dynamic sql code.
Here is a full dynamic solution whose result is close from your expected result and which relies on the creation of a user-defined composite type and on the standard functions jsonb_populate_record and jsonb_object_agg :
First you create the list of countries as a new composite type :
CREATE TYPE country_list AS () ;
CREATE OR REPLACE PROCEDURE country_list () LANGUAGE plpgsql AS
$$
DECLARE country_list text ;
BEGIN
SELECT string_agg(DISTINCT c.country || ' text', ',')
INTO country_list
FROM your_table
CROSS JOIN LATERAL jsonb_array_elements_text(countries) AS c(country) ;
EXECUTE 'DROP TYPE IF EXISTS country_list' ;
EXECUTE 'CREATE TYPE country_list AS (' || country_list || ')' ;
END ;
$$ ;
Then you can call the procedure country_list () just before executing the final query :
CALL country_list () ;
or even better call the procedure country_list () by trigger when the list of countries is supposed to be modified :
CREATE OR REPLACE FUNCTION your_table_insert_update()
RETURNS trigger LANGUAGE plpgsql VOLATILE AS
$$
BEGIN
IF EXISTS ( SELECT 1
FROM (SELECT jsonb_object_keys(to_jsonb(a.*)) FROM (SELECT(null :: country_list).*) AS a) AS b(key)
RIGHT JOIN jsonb_array_elements_text(NEW.countries) AS c(country)
ON c.country = b.key
WHERE b.key IS NULL
)
THEN CALL country_list () ;
END IF ;
RETURN NEW ;
END ;
$$ ;
CREATE OR REPLACE TRIGGER your_table_insert_update AFTER INSERT OR UPDATE OF countries ON your_table
FOR EACH ROW EXECUTE FUNCTION your_table_insert_update() ;
CREATE OR REPLACE FUNCTION your_table_delete()
RETURNS trigger LANGUAGE plpgsql VOLATILE AS
$$
BEGIN
CALL country_list () ;
RETURN OLD ;
END ;
$$ ;
CREATE OR REPLACE TRIGGER your_table_delete AFTER DELETE ON your_table
FOR EACH ROW EXECUTE FUNCTION your_table_delete() ;
Finally, you should get the expected result with the following query, except that the column label are lower case, and NULL is replacing false in the result :
SELECT (jsonb_populate_record(NULL :: country_list, jsonb_object_agg(lower(c.country), true))).*
FROM your_table AS t
CROSS JOIN LATERAL jsonb_array_elements_text(t.countries) AS c(country)
GROUP BY t
full test result in dbfiddle.