------------------------------------------
|team      | team2   | score | add_score |
------------------------------------------
|ManUnited | Chelsia |2:1    |           |
------------------------------------------
|Arsenal   | Everton |1:1    |           |
------------------------------------------
|StockCity |Liverpool|1:3    |           |
------------------------------------------

UPDATE `cal` SET dop_score = "win1" WHERE
 CONVERT( SUBSTRING(score, 1, 1), SIGNED ) > CONVERT( SUBSTRING(score, 3, 1), SIGNED )

UPDATE cal
SET dop_score = CASE WHEN SUBSTRING_INDEX(score, ':', 1) > SUBSTRING_INDEX(score, ':', -1)
                          THEN 'win1'
                     WHEN SUBSTRING_INDEX(score, ':', 1) < SUBSTRING_INDEX(score, ':', -1)
                          THEN 'win2'
                          ELSE 'draw'
                END
WHERE dop_score IS NULL -- зачем менять уже заполненное?
  AND LOCATE(':', score) -- а вдруг счёт ещё не заполнен?