Select *, sqrt((a.x-b.x)^2 + (a.y-b.y)^2) as Distance
from MyData a
Join MyData b
on a.x <> b.x and a.y <> b.y

Select top 1 *, sqrt((a.x-b.x)^2 + (a.y-b.y)^2) as Distance
from MyData a
Join MyData b
on a.x <> b.x and a.y <> b.y
order by Distance desc