skip if latter value equal

Hi list,

I was wondering if it was possible for a field in SQL query to return NULL
if latter value is exactly the same ? - for given ORDER BY clause, I guess.
For example, query returns:

xxyy 1 4 true
xxyy 5 7 true
xxyy 21 8 true
yyzz 5 1 false
yyzz 7 7 false
yyzz 8 34 false

I'd like the output to be:

xxyy 1 4 true
NULL 5 7 NULL
NULL 21 8 NULL
yyzz 5 1 false
NULL 7 7 NULL
NULL 8 34 NULL

Is there any magical trick to achieve this ?

regards
mk