How to avoid mutating table error while using a trigger? - sql

Question

I need to insert rows in a Table_A, with values from Table_B on UPDATE of Table_B.
table_A (
station int,
state varchar(20),
CONSTRAINT pk_table_a PRIMARY KEY(station, state),
CONSTRAINT fk_table_A FOREIGN KEY (station, state)
REFERENCES table_B (station, state)
)
table_B (
station int,
state varchar(20),
player int,
date_sent Date DEFAULT NULL
CONSTRAINT pk_table_b PRIMARY KEY(station, state, player)
)
Now, my triggers needs to add a row to table_A (station, state), when all the dates for these (station, state) become NOT NULL in table_B.
Here's my actual which causes mutating table error:
CREATE OR REPLACE TRIGGER add_stations_sent
AFTER INSERT OR UPDATE ON "TABLE_B"
FOR EACH ROW
WHEN (NEW.DATE_SENT IS NOT NULL)
DECLARE
nb_stations_null number;
BEGIN
SELECT COUNT(1)
INTO nbr_stations_null
FROM "TABLE_B"
WHERE "TABLE_B".STATE = :NEW.STATE AND
"TABLE_B".STATION <> :NEW.STATION AND
"TABLE_B".DATE_SENT IS NULL;
IF (nb_stations_null = 0) THEN
INSERT INTO "TABLE_A" VALUES (:NEW.STATION, :NEW.STATE);
END IF;
END;

Answer 1

Something like this will defer the processing to AFTER STATEMENT level and thus allow to you run queries
create or replace
trigger avoid_mutate
for insert or update on table_b
compound trigger
l_station sys.odcivarchar2list := sys.odcivarchar2list();
l_state sys.odcivarchar2list := sys.odcivarchar2list();
before each row is
begin
l_station.extend;
l_state.extend;
l_station(l_station.count) := :new.station;
l_state(l_state.count) := :new.state;
end before each row;
after statement is
declare
nb_stations_null number;
begin
for i in 1 .. l_station.count loop
SELECT COUNT(1)
INTO nbr_stations_null
FROM "TABLE_B"
WHERE "TABLE_B".STATE = l_state(i) AND
"TABLE_B".STATION <> l_station(i) AND
"TABLE_B".DATE_SENT IS NULL;
IF (nb_stations_null = 0) THEN
INSERT INTO "TABLE_A" VALUES (l_station(i), l_state(i));
END IF;
end after statement;
end;
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