PostgreSQL and Libpq - Create trigger that prints a line when requirement is not met

Question

I really need help on this one, it's for my university project and I'm stuck.

The big idea is to create a trigger on a table that if the value requirement is not met it prints a message to the user and doesn't allow the insert statement to proceed.

I already have a doSQL(conn, "query") function that sends the query to the database and prints any results (if any). Any SELECTs, UPDATEs or INSERTs work just fine with this function, however, I have a problem with multiple lines queries.

For example this:

doSQL(conn, "CREATE OR REPLACE FUNCTION najnizsza_krajowa() RETURNS trigger AS $$ BEGIN IF NEW.pensja<1500 THEN SELECT 'Pensja poniżej najniższej krajowej'; END IF; RETURN NEW; END; $$ LANGUAGE 'plpgsql' ");

returns an error saying ERROR: syntax error at or near "END" indicating at END IF; (not the later END). After this I have a create trigger statement

doSQL(conn, "CREATE TRIGGER pensja_check BEFORE INSERT ON pracownik FOR EACH ROW EXECUTE PROCEDURE najnizsza_krajowa();");

which I guess should work, but as I keep messing something with my function it only says that the function is missing. What do I do wrong with the function?

EDIT: The doSQL function, as requested:

void doSQL(PGconn *polaczenie, char *komenda){
PGresult *rez;

printf("%s\n", komenda);

rez = PQexec(polaczenie, komenda);
printf("Status: %s\n", PQresStatus(PQresultStatus(rez)));
printf("Komunikaty: %s\n", PQresultErrorMessage(rez));

switch(PQresultStatus(rez)) {
case PGRES_TUPLES_OK:
printTuples(rez);
break;
case PGRES_FATAL_ERROR:
printf("Coś nie tak!");
break;
}
PQclear(rez);
}

Sorry for some polish names, hope this won't matter much. Thanks in advance for any help, this is really important for me.

Answer 1

The unescaped single quotes you detected are not the problem. You cannot SELECT in a plpgsql function without a target. You would use PERFORM for that, but that's still not what you want. RAISE an EXCEPTION:

To do this:

prints a message to the user and doesn't allow the insert statement to proceed.

Use instead:

CREATE OR REPLACE FUNCTION najnizsza_krajowa()
  RETURNS trigger AS
$$
BEGIN
IF NEW.pensja < 1500 THEN
    RAISE EXCEPTION 'Pensja poniżej najniższej krajowej';
END IF;
RETURN NEW;
END;
$$ LANGUAGE plpgsql

Or you could RAISE a WARNING and RETURN NULL; to avoid the rollback.

And do not quote the language name: 'plpgsql'. It's an identifier, not a string.