Consider the following table

```
id   attribute
1     a
1     a
1     b
2     a
2     a
3     c
4     a

```

I want to select the ids that have attribute of 'a' only, ie 2 and 4.
Cant select 1 because 1 has 'a' and 'b', cant select 3 because it has 'c' only. We select 2 and 4 because it has 'a' value only.