Using groupBy to improve my Select (select count) query - sql-server

Question

Let's say we have this and want to see all Tasks, that havent been done yet and an additional column showing how many open Tasks there are left for this customer.
I have a table like this in my database:
+------------+--------------------------+-------+
| CustomerID | Task | Done |
+------------+--------------------------+-------+
| 1 | CleanRoom | False |
| 1 | Cleandishes | True |
| 1 | WashClothes | False |
| 2 | TakeDogsOut | True |
| 2 | PlayWithKids | True |
| 3 | HaveFunWithMrSamplesWife | True |
| 3 | CleanMrSamplesCar | False |
+------------+--------------------------+-------+
I need this as returned table:
+------------+-------------------+-------------+
| CustomerID | Task | DoneOverAll |
+------------+-------------------+-------------+
| 1 | CleanRoom | 2 |
| 1 | WashClothes | 2 |
| 3 | CleanMrSamplesCar | 1 |
+------------+-------------------+-------------+
Perfect return table would be like this, but I can do that myself when I have the one above:
About this a question; Doing this will probably be a String combination task. Should I do this on the Select statement, or would it be more advisable to do that in the final application on the client computer?
+------------+-------------------+-------------+
| CustomerID | Task | DoneOverAll |
+------------+-------------------+-------------+
| 1 | CleanRoom | 1/3 |
| 1 | WashClothes | 1/3 |
| 3 | CleanMrSamplesCar | 1/2 |
+------------+-------------------+-------------+
I know I could go like
SELECT
a.CustomerID,
a.Task,
(
Select count(*) from myTable where
customerID = a.CustomerID and
done = False
) as DoneOverAll
FROM myTable as a
WHERE Done = False
But I think that this is very ineffective, since it would execute a Select Count for each row in my table. Is there a way to achieve this with a JOIN using groupBy or something? I'm not into GroupBy commands yet.
Okay I should have tried first. Came up with the following;
Select count(*), CustomerID from myTable group by CustomerID
All I need to do now is to get this into a join.
Okay, got it. Sorry again for not trying first!
SELECT
a.CustomerID,
a.Task,
b.cnt
FROM myTable as a
LEFT JOIN (select count(*) AS cnt, CustomerID FROM myTable GROUP BY CustomerID) as b on a.CustomerID = B.CustomerID
WHERE Done = False
Question left;
Perfect return table would be like this, but I can do that myself when I have the one above:
About this a question; Doing this will probably be a String combination task. Should I do this on the Select statement, or would it be more advisable to do that in the final application on the client computer?
+------------+-------------------+-------------+
| CustomerID | Task | DoneOverAll |
+------------+-------------------+-------------+
| 1 | CleanRoom | 1/3 |
| 1 | WashClothes | 1/3 |
| 3 | CleanMrSamplesCar | 1/2 |
+------------+-------------------+-------------+

Answer 1

I'm not sure why Done = False, but this is your logic. :-)
Here's what I would do, without the LEFT JOIN.
SELECT
a.CustomerID,
a.Task,
SUM(CASE WHEN a.Done = 'False' THEN 1 ELSE 0 END) DoneOverAll,
SUM(Case WHEN a.Done = 'True' THEN 1 ELSE 0 END) NotDone
FROM myTable as a
Group By a.CustomerID, a.Task

Answer 2

Do calculate separately .
;with tempfalse as(
SELECT
a.CustomerID,
a.Task,
count(*) as DoneOverAll
FROM myTable as a
WHERE Done = False
group by a.CustomerID, a.Task
)
, temptrue (
SELECT
a.CustomerID,
a.Task,
count(*) as total
FROM myTable as a
group by a.CustomerID, a.Task
)
SELECT
a.CustomerID,
a.Task,
cast(NULLIF(DoneOverAll,0) as varchar (10) ) + '/' + cast(NULLIF(b.total,0) as varchar (10) )
from temptrue as a left join tempfalse b
on a.CustomerID =a.CustomerID and
a.Task = b.Task