How can I get the set difference of two result sets?

Say I have a result set (just one column in each):

```
result1:
'a'
'b'
'c'

result2:
'b'
'c'

```

I want to minus what is in result1 by result2: result1 - result2 such that it equals:

```
 difference of result1 - result2:
 'a'

```