I have tables like below.

I would like to subtract all `customers` who has `product = A`

```
product  customer
A        A
B        A
C        B
B        B
A        C
B        C
.        .
.        .
.        . 

```

Therefore my desired result is like below

```
product  customer
A        A
B        A
A        C
B        C

```

grouping by `customer`and then subtract...

But I am novice and I didn't come up with.

If someone has any opinion,please let me know.

Thanks