更改受更新影响的行数

Question

I am trying to achieve here is to basically override 0 rows Updated , when UPDATE is issued in-case the actual PK/UK value doesn't exist in the table. This is what I have done:

Actual Table:

CREATE TABLE fdrgiit.vereine( 
team numeric(10) primary key, 
punkte int not null, 
serie int not null 
); 

Dummy Table:

CREATE TABLE fdrgiit.dummyup 
( 
id numeric(1) PRIMARY KEY, 
datetest timestamp 
); 

Inserted records in both the tables:

insert into vereine(team,punkte,serie) values(1, 50, 1); 
insert into vereine(team,punkte,serie) values(2, 30, 1); 
insert into vereine(team,punkte,serie) values(3, 25, 1); 
insert into vereine(team,punkte,serie) values(4, 37, 2); 
insert into dummyup values(1, now()); 

Created the following function and trigger:

create or replace function updateover() 
returns trigger as 
$BODY$ 
begin 
if EXISTS (select 1 FROM vereine WHERE team = new.team ) then 
RETURN NEW; 
else 
UPDATE fdrgiit.dummyup set datetest=now() where id=1; 
RETURN NULL; 
end if; 
end; 
$BODY$ 
LANGUAGE plpgsql; 

create trigger update_redundancy 
before update on vereine 
for each row 
execute procedure updateover() ;

But when I execute an UPDATE like this on the , I am still get 0 rows affected

update vereine set punkte=87 where team=5; 

Kindly review and please suggest if this is something that can be done.

Answer 1

You cannot trigger anything with an UPDATE that does not affect row as triggers are only fired for affected rows.

But you could wrap your alternative UPDATE into a function:

CREATE OR REPLACE FUNCTION updateover()
  RETURNS int AS
$func$
   UPDATE dummyup
   SET    datetest = now()
   WHERE  id = 1
   RETURNING 2;
$func$  LANGUAGE sql;

... and run your UPDATE nested like this:

WITH upd AS (
   UPDATE vereine
   SET    punkte = 87
   WHERE  team = 5  -- does not exist!
   RETURNING 1
   )
SELECT 1 FROM upd
UNION ALL
SELECT updateover()
LIMIT 1;

db<>fiddle here

If no row qualifies for an UPDATE , then 1st outer SELECT 1 FROM upd returns no row and Postgres keeps processing the 2nd SELECT updateover() . But if at least one row is affected, the final SELECT is never executed. Exactly what you want.

This updates dummyup one time if the UPDATE on vereine does not affect any rows; never several times. But that's ok, since now() is STABLE for the duration of the transaction.

Related:

• Return a value if no record is found