As the title suggests I need some help getting a running total in T-SQL. The problem is that the sum I need to do is the sum of a count: 

```
sum(count (distinct (customers))) 

```

Say if I ran the count alone, the result would be: 

```
Day | CountCustomers
----------------------
5/1  |      1
5/2  |      0
5/3  |      5

```

I need output with the sum to be :

```
Day | RunningTotalCustomers
----------------------
5/1  |      1
5/2  |      1
5/3  |      6

```

I've done running totals before using the `coalesce` method, but never with a count. I'm not sure how to do it now that I have the count.