how to raise an exception in postgres if there is no data on return

Question

I like to do construct a compare and swap operation so that it can be included in a batched transaction where if the cas statement fails, the rest of the statements will also fail.

This statement will update if time is 0 but doesn't throw an error if time is not 0 :

UPDATE account
SET time = '1'
WHERE id = (
    SELECT id FROM account WHERE id = 'id-0' and time = '0' LIMIT 1
) RETURNING id

I want to add something like this

IF NO DATA THEN
  RAISE EXCEPTION 'cas ';
END IF;

but not sure how to do it.

Answer 1

If you cannot use a plpgsql block for whatever reason, you can force a division by zero:

UPDATE account
SET time = '1'
WHERE id = (
    SELECT id FROM account WHERE id = 'id-0' and time = '0' LIMIT 1
) 
  AND 1/(select count(*) from account where id = 'id-0' and time = '0') is not null
RETURNING id;  

This is like stopping the bicycle I'm riding by jamming a stick in the front wheel spokes.

Answer 2

This works for me:

DO $$ 
DECLARE 
v_id TEXT; 
BEGIN 
    UPDATE "account" 
    SET (id, name) = ('t1', 'world') 
    WHERE id = ( SELECT id FROM "account" WHERE "id" = 't1' AND "name" = 'hello' LIMIT 1) 
    RETURNING id INTO v_id; 
    IF count(v_id) = 0 
        THEN RAISE EXCEPTION 'Cas Error for (account, t1)';
    END IF; 
END $$