Resetting Row number according to record data change

Question

I have got the set of data as follow

name  date  
x     2014-01-01
x     2014-01-02
y     2014-01-03
x     2014-01-04

and I'm trying to get this result

name  date           row_num
x     2014-01-01      1
x     2014-01-02      2
y     2014-01-03      1
x     2014-01-04      1

I have tried to run this query

select name,
    date,
    row_number () over (partition by name order by date) as row_num
from myTBL

but unfortunately I get this result

name  date           row_num
x     2014-01-01      1
x     2014-01-02      2
y     2014-01-03      1
x     2014-01-04      3

Please help.

Answer 1

You need to identify the groups of names that occur together. You can do this with a difference of row numbers. Then, use the grp for partitioning the row_number():

select name, date,
       row_number() over (partition by name, grp order by date) as row_num
from (select t.*,
             (row_number() over (order by date) -
              row_number() over (partition by name order by date)
             ) as grp
      from myTBL t
     ) t

For your sample data:

name  date         1st row_number   2nd      Grp
x     2014-01-01         1           1        0
x     2014-01-02         2           2        0
y     2014-01-03         3           1        2
x     2014-01-04         4           3        1

This should give you an idea of how it works.