First and last aggregate functions for Postgresql

first-and-last-aggregate-functions-psql.sql

-- Create a function that always returns the first non-NULL item
CREATE OR REPLACE FUNCTION public.first_agg ( anyelement, anyelement )
RETURNS anyelement LANGUAGE SQL IMMUTABLE STRICT AS $$
        SELECT $1;
$$;
 
-- And then wrap an aggregate around it
CREATE AGGREGATE public.FIRST (
        sfunc    = public.first_agg,
        basetype = anyelement,
        stype    = anyelement
);
 
-- Create a function that always returns the last non-NULL item
CREATE OR REPLACE FUNCTION public.last_agg ( anyelement, anyelement )
RETURNS anyelement LANGUAGE SQL IMMUTABLE STRICT AS $$
        SELECT $2;
$$;
 
-- And then wrap an aggregate around it
CREATE AGGREGATE public.LAST (
        sfunc    = public.last_agg,
        basetype = anyelement,
        stype    = anyelement
);

https://wiki.postgresql.org/wiki/First/last_(aggregate)