Using ALTER TABLE command in psql to add to a table

Question

I am trying to solve this extra credit problem for my homework. So we haven't learned about this yet, but I thought I would give it a try because extra credit is always good. I am trying to write an ALTER TABLE statement to add a column to a table. The full definition is here.

Use the ALTER TABLE command to add a field to the table called rank that is of type smallint. We’ll use this field to store a ranking of the teams. The team with the highest points value will be ranked number 1; the team with the second highest points value will be ranked number 2; etc. Write a PL/pgSQL function named update rank that updates the rank field to contain the appropriate number for all teams. (There are both simple and complicated ways of doing this. Think about how it can be done with very little code.) Then, define a trigger named tr update rank that fires after an insert or update of any of the fields {wins, draws}. This trigger should be executed once per statement (not per row).

The table that I am using is

  Table "table.group_standings"
    Column |         Type          | Modifiers

    --------+-----------------------+-----------
   team   | character varying(25)| not null
   wins   | smallint | not null
   losses | smallint | not null
   draws  | smallint | not null
   points | smallint | not null

  Indexes:
   "group_standings_pkey" PRIMARY KEY, btree (team)
  Check constraints:
   "group_standings_draws_check" CHECK (draws >= 0)
   "group_standings_losses_check" CHECK (losses >= 0)
   "group_standings_points_check" CHECK (points >= 0)
   "group_standings_wins_check" CHECK (wins >= 0)

heres my code

ALTER TABLE group_standings ADD COLUMN rank smallint;

I need help with writing the function to rank the teams