I have table like this...

```
cola colb
  11   r
  11   r
  11   r
  21   k
  21   k
  21   m
  31   x
  31   y
  31   z

```

I want to get:

```
cola count()
11    1
21    2
31    3

```

I want to count how many distinct values in col b exists in each col a.

Thanks!