Break update operation when function returns NULL value in PostgreSQL - sql

Question

Let' assume I have a table named mytable:
I have one function which returns text and sometime it can return NULL also. ( this is just demo function in real use case function is complex )
CREATE OR REPLACE FUNCTION parag_test (id text)
RETURNS text
LANGUAGE plpgsql
AS
$$
DECLARE
--- variables
BEGIN
IF(id= 'Ram') THEN
RETURN 'shyam';
ELSE
RETURN NULL;
END IF;
END
$$
I want to update mytable till the time when my function returns non NULL values. if it returns NULL value I want to break update operation that time.
if we use below update query it will not stops updating when function returns NULL
update mytable SET id = parag_test (id) ;
Table after triggering above query looks like :
But what my expectation of output is :
because when we try to update second row function parag_test will return NULL and I want to stop update operation there.
So is there any way in PostgreSQL for achieving that ?

Answer 1

If you do have a primary key (say row number) to your table, this approach could work. - https://dbfiddle.uk/?rdbms=postgres_14&fiddle=0350562961be16333f54ebbe0eb5d5cb
CREATE OR REPLACE FUNCTION parag_test()
RETURNS void
LANGUAGE plpgsql
AS
$$
DECLARE
a int;
i varchar;
BEGIN
FOR a, i IN SELECT row_num, id FROM yourtable order by row_num asc
LOOP
IF(i = 'Ram') THEN
UPDATE yourtable set id = 'shyam' where row_num = a;
END IF;
IF (i is null) then
EXIT;
END IF;
END LOOP;
END;
$$