validate if a role name exists on postgresql , then execute statement

check_role.sql

DO $$
BEGIN
    IF NOT EXISTS (SELECT 1 FROM pg_catalago.pg_roles WHERE rolname = 'reader') THEN
        CREATE ROLE reader;
    END IF;
END
$$;
GRANT SELECT ON tasks TO reader;

-- 

DO $$
BEGIN
    IF EXISTS(SELECT count(*) FROM pg_catalago.pg_roles WHERE rolename = 'adm') THEN
        GRANT SELECT ON testview TO adm;
    END IF;
END
$$;