If you mean that the score for 2013-08-02
is null then you should use IFNULL
SELECT GROUP_CONCAT(IFNULL(score, ''))
FROM DailyScore
WHERE team = '42'
AND Date IN('2013-08-01','2013-08-02','2013-08-03','2013-08-04')
If you mean there is not a record for 2013-08-02
for team 42
, you should look at using a calendar table
SELECT GROUP_CONCAT(IFNULL(score, ''))
FROM DailyScore
RIGHT JOIN Calendar ON Calendar.Date = DailyScore.Date
AND team = '42'
AND Calendar.Date IN('2013-08-01','2013-08-02','2013-08-03','2013-08-04')
demo
As a side note it seems like you could also use BETWEEN
SELECT GROUP_CONCAT(IFNULL(score, ''))
FROM DailyScore
WHERE team = '42'
AND Date BETWEEN '2013-08-01' AND '2013-08-04'