select k.client, k.date_differ, t. date,t.def

from (

select  t.application, t.date- LAG(t.date) OVER(ORDER BY client ASC) as date_differ

from TABLE t group by client,application, t.date) k

left join TABLE t on k.application=t.application

where k.date_differ>=15

group by client, differ,t.date,t.def ;