ROW_NUMBER query - sql

Question

I have a table:
Trip Stop Time
-----------------
1 A 1:10
1 B 1:16
1 B 1:20
1 B 1:25
1 C 1:31
1 B 1:40
2 A 2:10
2 B 2:17
2 C 2:20
2 B 2:25
I want to add one more column to my query output:
Trip Stop Time Sequence
-------------------------
1 A 1:10 1
1 B 1:16 2
1 B 1:20 2
1 B 1:25 2
1 C 1:31 3
1 B 1:40 4
2 A 2:10 1
2 B 2:17 2
2 C 2:20 3
2 B 2:25 4
The hard part is B, if B is next to each other I want it to be the same sequence, if not then count as a new row.
I know
row_number over (partition by trip order by time)
row_number over (partition by trip, stop order by time)
None of them will meet the condition I want. Is there a way to query this?

Answer 1

create table test
(trip number
,stp varchar2(1)
,tm varchar2(10)
,seq number);
insert into test values (1, 'A', '1:10', 1);
insert into test values (1, 'B', '1:16', 2);
insert into test values (1, 'B', '1:20', 2);
insert into test values (1 , 'B', '1:25', 2);
insert into test values (1 , 'C', '1:31', 3);
insert into test values (1, 'B', '1:40', 4);
insert into test values (2, 'A', '2:10', 1);
insert into test values (2, 'B', '2:17', 2);
insert into test values (2, 'C', '2:20', 3);
insert into test values (2, 'B', '2:25', 4);
select t1.*
,sum(decode(t1.stp,t1.prev_stp,0,1)) over (partition by trip order by tm) new_seq
from
(select t.*
,lag(stp) over (order by t.tm) prev_stp
from test t
order by tm) t1
;
TRIP S TM SEQ P NEW_SEQ
------ - ---------- ---------- - ----------
1 A 1:10 1 1
1 B 1:16 2 A 2
1 B 1:20 2 B 2
1 B 1:25 2 B 2
1 C 1:31 3 B 3
1 B 1:40 4 C 4
2 A 2:10 1 B 1
2 B 2:17 2 A 2
2 C 2:20 3 B 3
2 B 2:25 4 C 4
10 rows selected
You want to see if the stop changes between one row and the next. If it does, you want to increment the sequence. So use lag to get the previous stop into the current row.
I used DECODE because of the way it handles NULLs and it is more concise than CASE, but if you are following the text book, you should probably use CASE.
Using SUM as an analytic function with an ORDER BY clause will give the answer you are looking for.

Answer 2

select *, dense_rank() over(partition by trip, stop order by time) as sqnc
from yourtable;
Use dense_rank so you get all the numbers consecutively, with no skipped numbers in between.

Answer 3

I think this is more complicated than a simple row_number(). You need to identify groups of adjacent stops and then enumerate them.
You can identify the groups using a difference of row numbers. Then, a dense_rank() on the difference does what you want if there are no repeated stops on a trip:
select t.*,
dense_rank() over (partition by trip order by grp, stop)
from (select t.*,
(row_number() over (partition by trip order by time) -
row_number() over (partition by trip, stop order by time)
) as grp
from table t
) t;
If there are:
select t.*, dense_rank() over (partition by trip order by mintime)
from (select t.*,
min(time) over (partition by trip, grp, stop) as mintime
from (select t.*,
(row_number() over (partition by trip order by time) -
row_number() over (partition by trip, stop order by time)
) as grp
from table t
) t
) t;