Postgres 12

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CREATE TABLE summary (id int, "TransactionA" numeric, "TransactionB" numeric);
CREATE TABLE summary2 (id int, "TransactionA" numeric, "TransactionC" numeric NOT NULL);

CREATE TABLE



CREATE TABLE


SELECT attname
FROM   pg_attribute a
WHERE  attrelid = 'summary'::regclass
AND    NOT attisdropped
AND    attnum > 0
AND    attname IN ('TransactionA', 'TransactionB', 'TransactionC');


attname


TransactionA

TransactionB



SELECT 2


-- build complete query dynamically
CREATE OR REPLACE FUNCTION f_build_query(_tbl regclass, _columns json)
  RETURNS text AS
$func$
DECLARE
   _expr text;
BEGIN

SELECT INTO _expr
       string_agg (op || CASE WHEN attnotnull
                              THEN quote_ident(attname)
                              ELSE format('COALESCE(%I, 0)', attname) END
                 , '')
FROM  (
   SELECT j->>'name' AS attname
        , CASE WHEN j->>'op' = '-' THEN ' - ' ELSE ' + ' END AS op
   FROM   json_array_elements(_columns) j
   ) j
JOIN   pg_attribute a USING (attname)
WHERE  attrelid = _tbl
AND    NOT attisdropped
AND    attnum > 0;

IF NOT FOUND THEN
   RAISE EXCEPTION 'No column found!'; -- or more info
END IF;

RETURN
'SELECT *,' || _expr || ' AS "Account_balance"
FROM   ' || _tbl || '
ORDER  BY id;';

END
$func$  LANGUAGE plpgsql;

CREATE FUNCTION


SELECT f_build_query('summary', '[{"name":"TransactionA"}
                                , {"name":"TransactionB", "op": "-"}
                                , {"name":"TransactionC"}]');


f_build_query


SELECT *, + COALESCE("TransactionA", 0) - COALESCE("TransactionB", 0) AS "Account_balance"
FROM   summary
ORDER  BY id;



SELECT 1


SELECT f_build_query('summary2', '[{"name":"TransactionA"}
                                 , {"name":"TransactionB", "op": "-"}
                                 , {"name":"TransactionC"}]');


f_build_query


SELECT *, + COALESCE("TransactionA", 0) + "TransactionC" AS "Account_balance"
FROM   summary2
ORDER  BY id;



SELECT 1