Try

...
FROM mytable mt1
...
  AND NOT EXISTS (SELECT 1 FROM mytable mt2 WHERE mt1.family ref = mt2.individual_ref)
...

>
>I have a list of individuals with a key of individual_ref
>those individuals can be linked to a partner by family_ref
>
>so I would have a row
>
>individual_ref family ref
>58056 174279
>174279 58056
>
>I am selecting a random number of individual but I don't want partners to be selected
>
>How can I do that.
>
>Thanks
>
>Nick