Trigger function to UPDATE a column up on INSERT - sql

Question

I would like to setup a trigger function for a postgresql table, which should update a column with the data derived from another column.
Table:
CREATE TABLE day (
symbol varchar(20) REFERENCES another_table(symbol),
date date,
open NUMERIC(8,2),
high NUMERIC(8,2),
low NUMERIC(8,2),
close NUMERIC(8,2),
sma8 NUMERIC(8,2),
PRIMARY KEY(symbol, date));
Note: composite primary key.
Sample INSERT:
INSERT INTO day VALUES('ABC', '2019-03-19', 102.3, 110.0, 125.3, 120.4, 0);
INSERT INTO day VALUES('ABC', '2019-03-20', 112.3, 109.0, 119.3, 118.4, 0);
INSERT INTO day VALUES('DEF', '2019-03-19', 1112.3, 1100.0, 1155.3, 1120.4, 0);
INSERT INTO day VALUES('DEF', '2019-03-20', 1202.3, 1180.0, 1205.3, 1190.4, 0);
and so on.
The following trigger function works fine when the 'date' column is the only primary key and the table contains data pertaining to one 'symbol' only (i.e the table contains data of one particular symbol on various unique dates).
create or replace function update_sma8() RETURNS TRIGGER AS
$$
BEGIN
UPDATE day d SET sma8 = s.simple_mov_avg
FROM
(
SELECT sec.date,AVG(sec.close)
OVER(ORDER BY sec.date ROWS BETWEEN 7 PRECEDING AND CURRENT ROW) AS
simple_mov_avg FROM day sec
)s where s.date = NEW.date --The newly inserted date
AND d.date = s.date;
RETURN NULL;
END $$ language plpgsql;
Refer: SQL trigger function to UPDATE daily moving average upon INSERT
I would like to update 'sma8' column with the value derived by averaging the current 'close' value and the last 7 'close' values of one particular symbol ('date' varies i.e past data.). Likewise for other symbols.
Kindly guide me. Thank you.

Answer 1

You need to know how to filter rows by "symbol".
Add WHERE clause to filter.
WHERE sec.symbol = NEW.symbol
And then, you register the trigger.
CREATE TRIGGER day_insert
AFTER INSERT ON day
FOR EACH ROW
EXECUTE PROCEDURE update_sma8();
Make sure that the "sma8" column will be updated when a row is inserted.
Here is full code.
DROP FUNCTION public.update_sma8();
CREATE FUNCTION public.update_sma8()
RETURNS trigger
LANGUAGE 'plpgsql'
COST 100
VOLATILE NOT LEAKPROOF
AS $BODY$
BEGIN
UPDATE day d SET sma8 = s.simple_mov_avg
FROM
(
SELECT sec.date,AVG(sec.close)
OVER(ORDER BY sec.date ROWS BETWEEN 7 PRECEDING AND CURRENT ROW) AS
simple_mov_avg FROM day sec WHERE symbol = NEW.symbol
)s where s.date = NEW.date --The newly inserted date
AND d.date = s.date ;
RETURN NULL;
END $BODY$;
ALTER FUNCTION public.update_sma8()
OWNER TO postgres;

Answer 2

You may add PARTITION BY symbol and then use it in the where clause to calculate for each symbol.
create or replace function update_sma8() RETURNS TRIGGER AS
$$
BEGIN
UPDATE day d SET sma8 = s.simple_mov_avg
FROM
(
SELECT sec.date,sec.symbol,AVG(sec.close)
OVER( partition by symbol ORDER BY sec.date ROWS BETWEEN
7 PRECEDING AND CURRENT ROW) AS
simple_mov_avg FROM day sec
)s where s.date = NEW.date --The newly inserted date
AND d.date = s.date
AND d.symbol = s.symbol;
RETURN NULL;
END $$ language plpgsql;
DEMO