Oracle Analytic functions - resetting a windowing clause

Question

I have the following data set.

create table t1 (
  dept number,
  date1 date
);

Table created.

insert into t1 values (100, \'01-jan-2013\');
insert into t1         


        

Answer 1

This could have been a case for model clause, but unfortunately it dramatically underperforms on significant amount of rows compared to Gordon's query.

select dept, date1, rank from t1
model 
  dimension by ( row_number() over(order by date1) as rn )
  measures( 1 as rank, dept, date1 ) 
  rules ( 
    rank[1] = 1,
    rank[rn > 1] = 
    case dept[cv()] 
      when dept[cv()-1] then rank[cv()-1] + 1 
      else 1
     end
  )

http://www.sqlfiddle.com/#!4/fc339/132