SQL Server - Using Sum of Count - sql

Question

I am using SQL Server. The Formula I need to use is (Good-Bad)/Total_Responses. A Good is when the value is >=9 and a Bad is <=6.
The Data I have is:
DATE Q1 Q2
2012-03-04 9 9
2012-03-04 8 8
2012-03-04 7 9
2012-03-04 4 NA
2012-03-04 10 10
2012-03-04 8 3
2012-03-04 3 4
2012-03-04 2 6
2012-03-04 6 8
2012-03-04 NA 6
I know I am going to have to use a "CASE WHEN ISNUMERIC(Q1)=1" to make sure it does not use the NA values (They are stored as Nvarchar)
So the formula would end up being (5-8)/18=-0.16666
So overall I am trying to get the data to look like:
DATE Promotor_Score
2012-03-04 -0.16666
Thank You!

Answer 1

Does this do the trick?
SELECT Date,
CAST((SUM(CASE WHEN ISNUMERIC(Q1) != 1 THEN 0
WHEN CAST(Q1 AS int) >= 9 THEN 1
WHEN CAST(Q1 AS int) <= 6 THEN -1
ELSE 0 END)
+ SUM(CASE WHEN ISNUMERIC(Q2) != 1 THEN 0
WHEN CAST(Q2 AS int) >= 9 THEN 1
WHEN CAST(Q2 AS int) <= 6 THEN -1
ELSE 0 END)) AS float)
/ (SUM(CASE WHEN ISNUMERIC(Q1) != 1 THEN 0
ELSE 1 END)
+ SUM(CASE WHEN ISNUMERIC(Q2) != 1 THEN 0
ELSE 1 END))
FROM Questions
GROUP BY Date
Or if 'NA' is the only non-numeric value, test for it explicitly.

Answer 2

Buildin on #DavidM answer, I added float conversion and Q2 column awareness:
SELECT [date],
1e0 -- a float multiplier to avoid integer value
* SUM( 0
-- get the positive, negative or neutral from q1
+ CASE WHEN ISNUMERIC(q1) != 1 THEN 0
WHEN CAST(q1 AS int) >= 9 THEN 1
WHEN CAST(q1 AS int) <= 6 THEN -1
ELSE 0 END
-- get the positive, negative or neutral from q2
+ CASE WHEN ISNUMERIC(q2) != 1 THEN 0
WHEN CAST(q2 AS int) >= 9 THEN 1
WHEN CAST(q2 AS int) <= 6 THEN -1
ELSE 0 END
)
/ SUM( 0
-- get the number of valid questions from q1
+ CASE WHEN ISNUMERIC(q1) != 1 THEN 0 ELSE 1 END
-- get the number of valid questions from q2
+ CASE WHEN ISNUMERIC(q2) != 1 THEN 0 ELSE 1 END
)
FROM Questions
GROUP BY Date;