Subquery as string to new query in postgresql - sql

Question

I have this working static query:
SELECT
companyname,
companyid,
incorporationcountryid,
periodEndDate,
filingdate,
AVG(dataitemvalue) FILTER (WHERE menmonic = 'AT') as "AT",
AVG(dataitemvalue) FILTER (WHERE menmonic = 'COGS') as "COGS"
FROM
aaa3
GROUP BY
companyname, companyid,
incorporationcountryid, periodEndDate, filingdate
ORDER BY
companyname, periodEndDate
Now, I'd like to incorporate all mnemonics, not just 'AT' and 'COGS' and adjust my query dynamically. I'm relatively new to SQL, but I managed to come up with this query, that gets me all unique mnemonics in my table and creates my desired query as a string:
SELECT 'SELECT companyname,companyid,incorporationcountryid,periodEndDate,filingdate,'
|| STRING_AGG(DISTINCT CONCAT('AVG(dataitemvalue) FILTER (WHERE menmonic = "', menmonic,'") as "',menmonic,'"'),',')
|| ' FROM aaa3 GROUP BY companyname, companyid, incorporationcountryid, periodEndDate, filingdate ORDER BY companyname, periodEndDate;'
as menmonic FROM aaa2;
Q: Can I turn this string into a query?

Answer 1

The first idea that comes up in mind would be to store your string into a text variable and then execute it as a dynamic query using EXECUTE (see the manual).
But then comes the issue : the query to be executed has a variable number of columns depending on the number of menmonics in table aaa2.
So the problem is a bit more tricky and one possible solution is to create dynamically a composite type menmonic_list corresponding to the list of menmonics and then to use the jsonb_populate_record and jsonb_object_agg standard functions :
Creating the composite type menmonic_list by trigger each time a row is inserted or updated or deleted from table aaa2 :
CREATE OR REPLACE FUNCTION aaa2_AFTER_INSERT_UPDATE_DELETE ()
RETURNS trigger LANGUAGE plpgsql AS $$
DECLARE
str text ;
BEGIN
SELECT string_agg(DISTINCT quote_ident(menmonic) || ' numeric', ',')
INTO str
FROM aaa2 ;
DROP TYPE IF EXISTS menmonic_list ;
EXECUTE 'CREATE TYPE menmonic_list AS (' || str || ')' ;
RETURN NULL ;
END ; $$ ;
CREATE OR REPLACE TRIGGER aaa2_AFTER AFTER INSERT OR UPDATE OF menmonic OR DELETE ON aaa2
FOR EACH statement EXECUTE FUNCTION aaa2_AFTER_INSERT_UPDATE_DELETE () ;
The final query is :
SELECT a.companyname,a.companyid,a.incorporationcountryid,a.periodEndDate,a.filingdate,
(jsonb_populate_record(null :: menmonic_list, jsonb_object_agg(a.key, a.value))).*
FROM
( SELECT companyname,companyid,incorporationcountryid,periodEndDate,filingdate,
menmonic AS key, AVG(dataitemvalue) AS value
FROM aaa3
GROUP BY companyname, companyid, incorporationcountryid, periodEndDate, filingdate, menmonic
) AS a
GROUP BY a.companyname, a.companyid, a.incorporationcountryid, a.periodEndDate, a.filingdate
ORDER BY a.companyname, a.periodEndDate;
see the test result in dbfiddle

Answer 2

I was able to come up with a compact solution myself:
DO $$
DECLARE
menmonic_list text[];
query text;
menmonic_unique text;
BEGIN
SELECT array_agg(DISTINCT menmonic) INTO menmonic_list
FROM aaa3;
query := 'SELECT companyname, companyid, incorporationcountryid, periodEndDate, filingdate';
FOREACH menmonic_unique IN ARRAY menmonic_list
LOOP
query := query || ', AVG(dataitemvalue) FILTER (WHERE aaa3.menmonic = ''' || menmonic_unique || ''') AS "' || menmonic_unique || '"';
END LOOP;
query := query || ' FROM aaa3 GROUP BY companyname, companyid, incorporationcountryid, periodEndDate, filingdate ORDER BY companyname, periodEndDate';
EXECUTE query;
END $$;